DevTools Workflow - Move Fast and Do Stuff

I’m starting my second week on the Firefox DevTools team and one of the most interesting things I’m working on is my own process.

The biggest bottleneck is the combination of compiling firefox and using feature branches. I like to have a couple feature branches so that I can asynchronously get feedback on bugs, run tests, and generally multitask. This can be a pain when compiling can take awhile.

Always be building

A full Firefox build takes upwards of ten minutes. That’s a productivity nightmare. An incremental build takes around a minute, which is significantly better.

In total, there’s a list of steps that are often repeated:

  1. build firefox ./mach build (faster)
  2. run firefox ./mach run
  3. go to a page to debug
  4. open the correct toolbox tool
  5. open the browser toolbox or web console
  6. start debugging

This process can quickly begin to feel repetitive, but there are some easy steps you can take to automate them away.

Firefox AutoRun

@bgrins wrote a great bash function ffrun which will build and run Firefox in a loop. The script automates steps 1 and 2. What I like about ffrun is that building and running the browser begins to feel like running tests in the background, it’s always there and upto-date.


  • You should add ffrun to your path and run chmod +x.
  • You can edit ffrun to add --jsconsole to launch the webconsole
  • You can edit ffrun to add --jsdebugger to launch the browser toolbox. There’s currently a regression where breakpoints won’t catch in the toolbox with this option.
  • You should use your profile so that the new browser has your favorite debugging settings.
# Start firefox in a loop, allowing you to quit and have it auto rebuild devtools and restart.

if [ -f mach ];
  echo "mach exists.  Starting loop."
  while true
     ./mach build faster
    #./mach run --devtools --jsconsole -P dev
    #./mach run --devtools --jsdebugger -P dev
    ./mach run --devtools -P dev
  echo "mach does not exist in current directory.  Bailing out."

Further Improvements:

ffrun gets us most of the way there, but there are still a couple of manual steps in the process. Here are a couple of other tricks that will help:

  • Always have a default page. You can set your current page to the default page in general preferences. I’m trying to use @bgrin’s demos project as a baseline for good test cases.
  • Don’t show the connection nag The connection nag is the popup that prompts you to accept the browser toolbox’s remote connection. You can suppress this nag by going to about:config and setting devtools.debugger.prompt-connection to false.
  • Suppress the restore page This is the page that shows up when Firefox quit unexpectedly. It shows a list of open tabs, which is nice when you’re browsing but not great when you’re debugging and want to automatically go to the default page. Fortunately this page can be suppressed by going to about:config and setting browser.sessionstore.resume_from_crash to false.

Balls in the air: Multitaksing with Feature Branches

One of the biggest pain points with having a time intensive build step is managing multiple branches. The problem is that often two branches are different enough to force full browser build, which is a great way to lose your focus.

@lclark found a great solution with git work trees. Work trees are an esoteric feature of git that lets you mount a feature branch in a directory. This makes cd path/to/feature-a synonymous with git checkout feature-a. The big win here is that we can now mount multiple features to different directories. So, if I’m working on two features A and B I’ll have the two folders worktrees/feature-a and worktrees/feature-b. Then, when I want to work on A, I’ll cd into the folder, run the tests, build the firefox, no big deal. If I want to jump to feature B, I can cd there as well. Jumping between branches no longer means re-building firefox from scratch.

I wrote two quick functions (branch-work, branch-work-rm) for creating and removing worktrees.

alias bw="branch-work"
function branch-work() {

  # create worktree directory if it doesn't exist
  if [ ! -d $DIR ]; then
    git worktree add -b $1 $DIR origin/fx-team

  cd ~/src/mozilla/worktrees/$1
alias bwrm="branch-work-rm"
function branch-work-rm() {

  # remove worktree directory if it exists
  if [ -d $DIR ]; then
    git bd $1;
    rm $DIR;

When I run branch-work foo three things happen:

  1. a new directory worktrees/foo,
  2. a new branch foo
  3. a new worktree foo

When I run branch-work foo a second time, it just navigates to worktrees/foo

When I run branch-work-rm foo it removes the foo worktree and branch.


There are definitely productivity costs associated with developing and debugging the browser, but I’ve found that if you’re creative about your process that a lot of these pains can be mitigated.

22 Mar 2016