Debugging On Windows

This document explains how to debug Gecko based applications such as Firefox, Thunderbird, and SeaMonkey on Windows using the Visual Studio IDE.

If VS and your Gecko application hang shortly after you launch the application under the debugger, see Problems Loading Debug Symbols.

Ways to start the debugger

First of all, it’s necessary to install a Visual Studio extension to be able to follow child processes as they are created. Firefox, in general, and even in non-e10s mode, does not start the main process directly, it starts it via a Launcher Process. This means that Visual Studio will only attach to the first process it finds, and will not hit any break-point (and even notifies you that it cannot find their location). Microsoft Child Process Debugging Power Tool allows automatically attaching to child processes, such as Web Content process, GPU process, etc. Enable it by going its configuration menu in “Debug > Other debugging targets > Child process debugging settings”, and ticking the box.

If you have followed the steps in Building Firefox for Windows and have a local debug build, you can execute this command from same command line.

./mach run --debug

It would open Visual Studio with Firefox’s run options configured. You can click “Start” button to run Firefox then, already attached in the debugger.

Alternatively, if you have generated the Visual Studio solution, via ./mach build-backend -b VisualStudio, opening this solution allows you to run firefox.exe directly in the debugger. To make it the startup project, right click on the project and select Set As Startup Project. It appears bold when it’s the case. Breakpoints are kept across runs, this can be a good way to debug startup issues.

Run the program until you hit an assertion. You will get a dialog box asking if you would like to debug. Hit “Cancel”. The MSDEV IDE will launch and load the file where the assertion happened. This will also create a Visual Studio Mozilla project in the directory of the executable by default.

Attach the debugger to an existing Mozilla process. In the Visual Studio, select Debug > Attach to Process. If you want to debug a content process, you can hover on the tab of page you want to debug, which would show the pid. You can then select the process from dialog opened from “Attach to Process”. You can open about:processes to see the pid for all subprocesses, including tabs but also GPU, networking etc. For more information, see Attach to Running Processes with the Visual Studio Debugger.

Starting an MSIX installed Firefox with the debugger. In Visual Studio, select Debug -> Other Debug Targets -> Debug Installed App Package. In the dialog, select the installed Firefox package you wish to debug and click “Start”.

Debugging Release and Nightly Builds

Refer to the steps to use the Mozilla symbol server and source server

Creating a Visual Studio project for Firefox

Please refer to this.

Changing/setting the executable to debug

To change or set the executable to debug, go to Project > Properties > Debugging > Command. (As of Visual Studio 2022.)

It should show the executable you are debugging. If it is empty or incorrect, manually add the correct path to the executable.

Command line parameters and environment variables

To change or set the command line options, go to Project > Properties > Debugging > Command Arguments.

Some common options would be the URL of the file you want the browser to open as soon as it starts, starting the Profile Manager, or selecting a profile. You can also redirect the console output to a file (by adding “> filename.txt” for example, without the quotes).

Customizing the debugger’s variable value view

You can customize how Visual Studio displays classes in the variable view. By default VS displays “{…}” and you need to click the small + icon to expand the members. You can change this behaviour, and make Visual Studio display whatever data member you want in whatever order, formatted however you like instead of just “{…}”.

You need to locate a file called “gecko.natvis” under toolkit/library. The file contains a list of types and how they should be displayed in the debugger. It is XML and after a little practice you should be well on your way.

To understand the file in detail refer to Create custom views of C++ objects in the debugger using the Natvis framework

The file already comes with a number of entries that will make your life easier, like support for several string types. If you need to add a custom type, or want to change an existing entry for debugging purposes, you can easily edit the file. For your convenience it is included in all generated Visual Studio projects, and if you edit and save it within Visual Studio, it will pick up the changes immediately.

Handling multiple processes in Visual Studio

Turn off “Break all processes when one process breaks” to single step a single process.

Turning off “Break all processes when one process breaks” adds “Step Into Current Process”, “Step Over Current Process” and “Step Out Current Process” to the “Debug” menu.

To single step a single process with the other processes paused:

  • Turn on “Break all processes when one process breaks”

  • Hit a breakpoint which stops all processes

  • Turn off “Break all processes when one process breaks”

  • Now using “Step Into Current Process” will leave the other processes stopped and just advance the current one.

Obtaining stdout and other FILE handles

Running the following command in the Command Window in Visual Studio returns the value of stdout, which can be used with various debugging methods (such as nsGenericElement::List) that take a FILE* param:

Debug.EvaluateStatement {,,msvcr80d}(&__iob_func()[1])

(Alternatively you can evaluate {,,msvcr80d}(&__iob_func()[1]) in the Immediate window)

Similarly, you can open a file on the disk using fopen:

>Debug.EvaluateStatement {,,msvcr80d}fopen("c:\\123", "w")
0x10311dc0 { ..snip.. }
>Debug.EvaluateStatement ((nsGenericElement*)0x03f0e710)->List((FILE*)0x10311dc0, 1)
>Debug.EvaluateStatement {,,msvcr80d}fclose((FILE*)0x10311dc0)

Note that you may not see the debugging output until you flush or close the file handle.


There are basically two ways to disable assertions. One requires setting an environment variable, while the other affects only the currently running program instance in memory.

Environment variable

There is an environment variable that can disable breaking for assertions. This is how you would normally set it:


The environment variable takes also other values besides warn, see XPCOM_DEBUG_BREAK for more details.

Note that unlike Unix, the default for Windows is not warn, it’s to pop up a dialog. To set the environment variable for Visual Studio, use Project > Properties > Debugging > Environment and click the little box. Then use


Changing running code

You normally shouldn’t need to do this (just quit the application, set the environment variable described above, and run it again). And this can be dangerous (like trashing your hard disc and corrupting your system). So unless you feel comfortable with this, don’t do it. You have been warned!

It is possible to change the interrupt code in memory (which causes you to break into debugger) to be a NOP (no operation).

You do this by running the program in the debugger until you hit an assertion. You should see some assembly code. One assembly code instruction reads “int 3”. Check the memory address for that line. Now open memory view. Type/copy/drag the memory address of “int 3” into the memory view to get it to update on that part of the memory. Change the value of the memory to “90”, close the memory view and hit “F5” to continue.

Automatically handling ASSERTIONS without a debugger attached

When an assertion happens and there is not a debugger attached, a small helper application (`windbgdlg.exe </En/Automatically_Handle_Failed_Asserts_in_Debug_Builds>`__) is run. That application can automatically select a response to the “Do you want to debug” dialog instead of prompting if you configure it, for more info, see `windbgdlg.exe </En/Automatically_Handle_Failed_Asserts_in_Debug_Builds>`__.

Debugging optimized builds

To effectively debug optimized builds, you should enable debugging information which effectively leaves the debug symbols in optimized code so you can still set breakpoints etc. Because the code is optimized, stepping through the code may occasionally provide small surprises when the debugger jumps over something.

You need to make sure this configure parameter is set:

ac_add_options --enable-debug

You can also choose to include or exclude specific modules.

Console debugging

When printing to STDOUT from a content process, the console message will not appear on Windows. One way to view it is simply to disable e10s (./mach run --disable-e10s) but in order to debug with e10s enabled one can run

./mach run ... 2>&1 | tee

It may also be necessary to disable the content sandbox (MOZ_DISABLE_CONTENT_SANDBOX=1 ./mach run ...).

Running two instances of Mozilla simultaneously

You can run two instances of Mozilla (e.g. debug and optimized) simultaneously by setting the environment variable MOZ_NO_REMOTE:


Or, starting with Firefox 2 and other Gecko 1.8.1-based applications, you can use the -no-remote command-line switch instead (implemented in bug 325509).

You can also specify the profile to use with the -P profile_name command-line argument.

Debugging JavaScript

You can use helper functions from nsXPConnect.cpp to inspect and modify the state of JavaScript code from the MSVS debugger.

For example, to print current JavaScript stack to stdout, evaluate this in Immediate window:


Visual Studio will show you something in the quick watch window, but not the stack, you have to look in the OS console for the output.

Also this magical command only works when you have JS on the VS stack.

Debugging minidumps

See debugging a minidump.

Problems post-mortem debugging on Windows 7 SP1 x64?

If you attempt to use NS_DebugBreak etc to perform post-mortem debugging on a 64bit Windows 7, but as soon as you try and continue debugging the program crashes with an Access Violation, you may be hitting a Windows bug relating to AVX support. For more details, including a work-around see this blog post or this social.msdn thread. (And just in-case those links die, the work-around is to execute

bcdedit /set xsavedisable 1

from an elevated command-prompt to disable AVX support.)

Got a tip?

If you think you know a cool Mozilla debugging trick, feel free to discuss it with #developers and then post it here.