Skip to main content

Debugging Firefox Devtools in 2016

· 3 min read

Not like previous article about general workflow on Mozilla Gecko project, This article is like a cookbook that I encountered during debugging Firefox devtools (mostly around inspector and network monitor).

Remote debugging

According to MDN, first of all you need to go through Developer > Toggle Tools > Toolbox Options > Advance settings to check 2 options:

  • Enable browser chrome and add-on debugging toolboxes
  • Enable remote debugging

Then use Shift+Option+Command+i on Mac or Ctrl+Shift+i on linux to open the remote debugger.

Disable autohide in popup

Once I'm developing toolbar web extensions or any popup windows, I feel very appreciate this feature.

Boost build time via set mozconfig artifact

Add these settings in mozconfig file

# Enable debug version of the pre-build binary artifact
export MOZ_DEBUG="1"

# Automatically download and use compiled C++ components:
ac_add_options --enable-artifact-builds

# Write build artifacts to:
mk_add_options MOZ_OBJDIR=./objdir-frontend-debug-artifact

If you just develop frontend UI(JS, CSS, XUL, HTML), the artifact build prebuild the C++ and the static part of the code so you can download and link it automatically during mach build, it saves you tremendous build time and helps you move quicker.

You can use the command

./mach build faster

to further boost the build time.

Enable React and other JS debugging message

Add these settings in mozconfig file

ac_add_options --enable-debug-js-modules

It will turn on React and other JS debugging message. Note that will influence performance while running.

Open devtools and chrome debugger with website

You may not know its possible to open a website with opened devtool with the command:

./mach run --devtools --jsdebugger

Test on Try server

Mozilla Try Server let you run tests on all supported platforms. You can pick which test set and which platform to run by providing command arguments. Here's what I used for test devtools related bugs on Windows, Mac, and Linux.

try: -b do -p linux64,macosx64,win64 -u mochitest-dt,mochitest-e10s-devtools-chrome -t none --artifact

Add --artifact argument will both save total running time and computing resources.

Tracing Code

Via global search in editor

Many modern editor ex sublime, visual studio code provide great support of global search. Ctrl + Shift + f is your good friend.

Via DXR or searchfox

DXR and searchfox is a code search and navigation tool for large projects like Mozilla-center. It supports full-text and regex searches as well as structural queries.

ES6 nits

Do not use forEach with yield

forEach is a good way to loop array, but is not compatible with the generator(because the need of implement iterator interface), so if you use yield inside of forEach loop, it will not behave as you expect. The better choice is use for..of loop or map instead.

Try to be functional

The powerful => arrow syntax sugar is not just for replacing bind(this), it gives the developer a clear way to write more compact code.

For example

promise.then(function(a , b) {
return a + b;

can be written as

promise.then((a, b) => a + b);