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.

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!"));

Enabling Logging

The log level for a module is controlled by setting an environment variable before launching the application. It can also be adjusted by setting prefs. By default all logging output is disabled.

set MOZ_LOG="example_logger:3"

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.

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.

rotate: N

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’.

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

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

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 special boolean prefs logging.config.sync and logging.config.add_timestamp can be used to control the sync and timestamp properties described above.


A sandboxed content process cannot write to stderr or any file. The easiest way to log these processes is to disable the content sandbox by setting the preference security.sandbox.content.level to 0. 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.

Redirecting logging output to a file

Logging output can be redirected to a file by passing its path via an environment variable.


By default logging output goes to stderr.

set MOZ_LOG_FILE="log.txt"

The rotate and append options described above only apply when logging to a file.

The special pref logging.config.LOG_FILE can be set at runtime to change the log file being output to.

Logging Rust

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.