Hugo Peixoto

Recompiling firefox to change some keyboard shortcuts

Published on July 14, 2020

I’m trying to migrate from Chromium to Firefox.

One of the things that caught me by surprise is that the keyboard shortcuts to select the n-th tab are different. In chromium, they’re Ctrl+1 through Ctrl+9, while in firefox they’re Alt+1 through Alt+9. These shortcuts conflict with the ones I’m using in sway, where they’re used to navigate between workspaces.

This is not fixable via a setting change, so I decided to build a custom version of firefox. Since I’m using debian, this should be straightforward:

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$ sudo apt build-dep firefox    # install build dependencies
$ apt source firefox            # download source package
$ cd firefox-78.0.2/
$ make-necessary-changes-to-files
$ dpkg-source --commit          # create a patch file and changes version
$ dpkg-buildpackage -us -uc     # build the deb files

I was having some trouble finding what changes to make, though, and firefox’s codebase is not exactly small. tokei reports ~200k files and ~27M lines of code. acking didn’t help much, so I cloned the source repository to see if I could find something through the commit messages. Firefox uses mercurial, but there’s an official git mirror:

https://developer.mozilla.org/en-US/docs/Mozilla/Git

The current official git mirror of the Firefox code base (also known as “gecko” or “mozilla-central”) can be found at https://github.com/mozilla/gecko-dev.

After some tries, I found this:

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$ git log --grep "Ctrl+9"
commit 032ccdebf21b0e8493a28c4bee3ed759f87c5a40
Author: steffen.wilberg%web.de <steffen.wilberg%web.de>
Date:   Wed Jun 7 21:40:07 2006 +0000

    Bug 340553: Document that Ctrl+9 focuses last tab. p=zeniko@gmail.com, r=me

commit ce743abf4705fdbbf7695be3b929506f5563d183
Author: gavin%gavinsharp.com <gavin%gavinsharp.com>
Date:   Tue Jun 6 16:28:09 2006 +0000

    Bug 338348: Make Ctrl+9 select the last tab instead of the ninth tab, patch
    by Simon Bünzli <zeniko@gmail.com>, r=mconnor

The second commit was a good starting point. The code had changed a lot since that commit in 2006, but I was able to track it down by looking to the files history with a combination of git log -p <filename> and git show <commit-sha>. I ended up doing the following change:

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--- firefox-78.0.2.orig/browser/base/content/browser-sets.inc
+++ firefox-78.0.2/browser/base/content/browser-sets.inc
@@ -315,11 +315,7 @@
     <key id="key_undoCloseTab" command="History:UndoCloseTab" data-l10n-id="tab-new-shortcut" modifiers="accel,shift"/>
     <key id="key_undoCloseWindow" command="History:UndoCloseWindow" data-l10n-id="window-new-shortcut" modifiers="accel,shift"/>

-#ifdef XP_GNOME
-#define NUM_SELECT_TAB_MODIFIER alt
-#else
 #define NUM_SELECT_TAB_MODIFIER accel
-#endif

 #expand    <key id="key_selectTab1" oncommand="gBrowser.selectTabAtIndex(0, event);" key="1" modifiers="__NUM_SELECT_TAB_MODIFIER__"/>
 #expand    <key id="key_selectTab2" oncommand="gBrowser.selectTabAtIndex(1, event);" key="2" modifiers="__NUM_SELECT_TAB_MODIFIER__"/>

By removing the ifdef, it now always uses accel. It took roughly one hour to compile and it generated a bunch of deb files:

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$ ls -1 *.deb
firefox-dbgsym_78.0.2-1.1_amd64.deb
firefox-l10n-ach_78.0.2-1.1_all.deb
firefox-l10n-af_78.0.2-1.1_all.deb
[...]
firefox-l10n-zh-cn_78.0.2-1.1_all.deb
firefox-l10n-zh-tw_78.0.2-1.1_all.deb
firefox_78.0.2-1.1_amd64.deb

I installed the last one via dpkg -i firefox_78.0.2-1.1_amd64.deb, restarted firefox, and it works!

Not sure how I’ll keep recompiling firefox every time there’s a new firefox version. This doesn’t seem like something that would be accepted upstream. I didn’t see any keyboard shortcuts on that file whose modifiers depend on a configuration setting.

It would probably have been easier to install an extension for this.

Here are some firefox bug reports and commits related to this keyboard shortcut that I found along the way: