Gecko Logging

A minimal C++ logging framework is provided for use in core Gecko code. It is enabled for all builds and is thread-safe.

This page covers enabling logging for particular logging module, configuring the logging output, and how to use the logging facilities in native code.

Enabling and configuring logging

Caveat: sandboxing when logging to a file

Sandboxed content processes (on all OSes) cannot write to files on disk, so it is recommended to log to the terminal, possibly by redirecting the output to a file.

If the sandbox has been disabled and/or the logging statement are coming from the parent process, MOZ_LOG_FILE will work as expected. Otherwise, logging to the terminal works as expected on macOS and Linux on desktop.

On Windows, you can still see child process messages by using DOS (not the MOZ_LOG_FILE variable defined below) to redirect output to a file. For example: MOZ_LOG=CameraChild:5 mach run >& my_log_file.txt will include debug messages from the camera’s child actor that lives in a (sandboxed) content process.

Another way to do this and have output in the terminal when developing is by redirecting stderr to stdout and then stdout to another process, for example like so:

MOZ_LOG=cubeb:4 ./mach run 2>&1 | tee

Logging to the Firefox Profiler

When a log statement is logged on a thread and the Firefox Profiler is profiling that thread, the log statements is recorded as a profiler marker.

This allows getting logs alongside profiler markers and lots of performance and contextual information, in a way that doesn’t require disabling the sandbox, and works across all processes.

The profile can be downloaded and shared e.g. via Bugzilla or email, or uploaded, and the logging statements will be visible either in the marker chart or the marker table.

While it is possible to manually configure logging module and start the profiler with the right set of threads to profile, about:logging makes this task a lot simpler and error-proof.

The MOZ_LOG syntax

Logging is configured using a special but simple syntax: which module should be enabled, at which level, and what logging options should be enabled or disabled.

The syntax is a list of terms, separated by commas. There are two types of terms:

  • A log module and its level, separated by a colon (:), such as example_module:5 to enable the module example_module at logging level 5 (verbose). This searchfox query returns the complete list of modules available.

  • A special string in the following table, to configure the logging behaviour. Some configuration switch take an integer parameter, in which case it’s separated from the string by a colon (:). Most switches only apply in a specific output context, noted in the Context column.

Special module name





Append new logs to existing log file.



Print each log synchronously, this is useful to check behavior in real time or get logs immediately before crash.



Print exactly what has been specified in the format string, without the process/thread/timestamp, etc. prefix.



Insert timestamp at start of each log line.



This limits the produced log files’ size. Only most recent N megabytes of log data
is saved. We rotate four log files with .0, .1, .2, .3 extensions. Note: this option
disables ‘append’ and forces ‘timestamp’.



Limit the log to N MB. Only work in append mode.



Prepend a simple header while distinguishing logging. Useful in append mode.



When profiling with the Firefox Profiler and log modules are enabled, capture the call
stack for each log statement.

This syntax is used for most methods of enabling logging.

Enabling Logging

Enabling logging can be done in a variety of ways:

  • via environment variables

  • via command line switches

  • using about:config preferences

  • using about:logging

The first two allow logging from the start of the application and are also useful in case of a crash (when sync output is requested, this can also be done with about:config as well to a certain extent). The last two allow enabling and disabling logging at runtime and don’t require using the command-line.

By default all logging output is disabled.

Enabling logging using about:logging

about:logging allows enabling logging by entering a MOZ_LOG string in the text input, and validating.

Options allow logging to a file or using the Firefox Profiler, that can be started and stopped right from the page.

Logging presets for common scenarios are available in a drop-down. They can be associated with a profiler preset.

It is possible, via URL parameters, to select a particular logging configuration, or to override certain parameters in a preset. This is useful to ask a user to gather logs efficiently without having to fiddle with prefs and/or environment variable.

URL parameters are described in the following table:




a logging preset


alias for preset


a string in MOZ_LOG syntax


alias for modules


a list of threads to profile, overrides what a profiler preset would have picked


alias for threads


either profiler or file


alias for output


a profiler preset

If a preset is selected, then threads or modules can be used to override the profiled threads or logging modules enabled, but keeping other aspects of the preset. If no preset is selected, then a generic profiling preset is used, firefox-platform. For example:


will profile the threads in the Media profiler preset, but will only log specific log modules (instead of the long list in the media-playback preset). In addition, it disallows logging to a file.

Enabling logging using environment variables

On UNIX, setting and environment variable can be done in a variety of ways

set MOZ_LOG="example_logger:3"
export MOZ_LOG="example_logger:3"
MOZ_LOG="example_logger:3" ./mach run

In the Windows Command Prompt (cmd.exe), don’t use quotes:

set MOZ_LOG=example_logger:3

If you want this on GeckoView example, use the following adb command to launch process:

adb shell am start -n org.mozilla.geckoview_example/.GeckoViewActivity --es env0 "MOZ_LOG=example_logger:3"

There are special module names to change logging behavior. You can specify one or more special module names without logging level.

For example, if you want to specify sync, timestamp and rotate:

set MOZ_LOG="example_logger:3,timestamp,sync,rotate:10"

Enabling logging usually outputs the logging statements to the terminal. To have the logs written to a file instead (one file per process), the environment variable MOZ_LOG_FILE can be used. Logs will be written at this path (either relative or absolute), suffixed by a process type and its PID. MOZ_LOG files are text files and have the extension .moz_log.

For example, setting:

set MOZ_LOG_FILE="firefox-logs"

can create a number of files like so:


respectively for a parent process of PID 96353 and a child process of PID 96354.

Enabling logging using command-line flags

The MOZ_LOG syntax can be used with the command line switch on the same name, and specifying a file with MOZ_LOG_FILE works in the same way:

./mach run -MOZ_LOG=timestamp,rotate:200,example_module:5 -MOZ_LOG_FILE=%TEMP%\firefox-logs

will enable verbose (5) logging for the module example_module, with timestamp prepended to each line, rotate the logs with 4 files of each 50MB (for a total of 200MB), and write the output to the temporary directory on Windows, with name starting with firefox-logs.

Enabling logging using preferences

To adjust the logging after Firefox has started, you can set prefs under the logging. prefix. For example, setting to 3 will set the log module foo to start logging at level 3.

The MOZ_LOG syntax can be used directly as well, by setting the preference logging.config.modules. All modules can be used but only the special string profilerstacks is supported.

A number of special prefs can be set as well, described in the table below:

Preference name


Preference value




Whether to clear all prefs under logging.



A path (relative or absolute)

The path to which the log files will be written.



Whether to prefix all lines by a timestamp.



Whether to flush the stream after each log statements.



When logging to the Firefox Profiler, whether to
include the call stack in each logging statement.

Enabling logging in Rust code

We’re gradually adding more Rust code to Gecko, and Rust crates typically use a different approach to logging. Many Rust libraries use the log crate to log messages, which works together with env_logger at the application level to control what’s actually printed via RUST_LOG.

You can set an overall logging level, though it could be quite verbose:

set RUST_LOG="debug"

You can also target individual modules by path:

set RUST_LOG="style::style_resolver=debug"


For Linux/MacOS users, you need to use export rather than set.


Sometimes it can be useful to only log child processes and ignore the parent

process. In Firefox 57 and later, you can use RUST_LOG_CHILD instead of RUST_LOG to specify log settings that will only apply to child processes.

The log crate lists the available log levels:

Log Level



Designates very serious errors.


Designates hazardous situations.


Designates useful information.


Designates lower priority information.


Designates very low priority, often extremely verbose, information.

It is common for debug and trace to be disabled at compile time in release builds, so you may need a debug build if you want logs from those levels.

Check the env_logger docs for more details on logging options.

Additionally, a mapping from RUST_LOG is available. When using the MOZ_LOG syntax, it is possible to enable logging in rust crate using a similar syntax:


will enable debug logging for all log statements in the crate rust_crate_name.

* can be replaced by a series of modules if more specificity is needed:


will enable debug logging for all log statements in the sub-module submodule of the module module of the crate rust_crate_name.

A table mapping Rust log levels to MOZ_LOG log level is available below:

Rust log level

MOZ_LOG level

Numerical value



















Enabling logging on Android, interleaved with system logs (logcat)

While logging to the Firefox Profiler works it’s sometimes useful to have system logs (adb logcat) interleaved with application logging. With a device (or emulator) that adb devices sees, it’s possible to set environment variables like so, for e.g. GeckoView_example:

adb shell am start -n org.mozilla.geckoview_example/.GeckoViewActivity --es env0 MOZ_LOG=MediaDemuxer:4

It is then possible to see the logging statements like so, to display all logs, including MOZ_LOG:

adb logcat

and to only see MOZ_LOG like so:

adb logcat Gecko:V '*:S'

This expression means: print log module Gecko from log level Verbose (lowest level, this means that all levels are printed), and filter out (S for silence) all other logging (*, be careful to quote it or escape it appropriately, it so that it’s not expanded by the shell).

While interactive with e.g. GeckoView code, it can be useful to specify more logging tags like so:

adb logcat GeckoViewActivity:V Gecko:V '*:S'

Enabling logging on Android, using the Firefox Profiler

Set the logging modules using about:config (this requires a Nightly build) using the instructions outlined above, and start the profile using an appropriate profiling preset to profile the correct threads using the instructions written in Firefox Profiler documentation’s dedicated page.

Bug 1803607 tracks improving the logging experience on mobile.

Working with MOZ_LOG in the code

Declaring a Log Module

LazyLogModule defers the creation the backing LogModule in a thread-safe manner and is the preferred method to declare a log module. Multiple LazyLogModules with the same name can be declared, all will share the same backing LogModule. This makes it much simpler to share a log module across multiple translation units. LazyLogLodule provides a conversion operator to LogModule* and is suitable for passing into the logging macros detailed below.

Note: Log module names can only contain specific characters. The first character must be a lowercase or uppercase ASCII char, underscore, dash, or dot. Subsequent characters may be any of those, or an ASCII digit.

#include "mozilla/Logging.h"

static mozilla::LazyLogModule sFooLog("foo");

Logging interface

A basic interface is provided in the form of 2 macros and an enum class.

MOZ_LOG(module, level, message)

Outputs the given message if the module has the given log level enabled:

  • module: The log module to use.

  • level: The log level of the message.

  • message: A printf-style message to output. Must be enclosed in parentheses.

MOZ_LOG_TEST(module, level)

Checks if the module has the given level enabled:

  • module: The log module to use.

  • level: The output level of the message.

Log Level

Numeric Value




Indicates logging is disabled. This should not be used directly in code.



An error occurred, generally something you would consider asserting in a debug build.



A warning often indicates an unexpected state.



An informational message, often indicates the current program state.



A debug message, useful for debugging but too verbose to be turned on normally.



A message that will be printed a lot, useful for debugging program flow and will probably impact performance.

Example Usage

#include "mozilla/Logging.h"

using mozilla::LogLevel;

static mozilla::LazyLogModule sLogger("example_logger");

static void DoStuff()
  MOZ_LOG(sLogger, LogLevel::Info, ("Doing stuff."));

  int i = 0;
  int start = Time::NowMS();
  MOZ_LOG(sLogger, LogLevel::Debug, ("Starting loop."));
  while (i++ < 10) {
    MOZ_LOG(sLogger, LogLevel::Verbose, ("i = %d", i));

  // Only calculate the elapsed time if the Warning level is enabled.
  if (MOZ_LOG_TEST(sLogger, LogLevel::Warning)) {
    int elapsed = Time::NowMS() - start;
    if (elapsed > 1000) {
      MOZ_LOG(sLogger, LogLevel::Warning, ("Loop took %dms!", elapsed));

  if (i != 10) {
    MOZ_LOG(sLogger, LogLevel::Error, ("i should be 10!"));