Draft: Bug 282: Move `--disable-eme` to OS- and architecture-specific mozconfigs

Merge Info

Related Issues

• #282 (closed)
• tor-browser#42501 (closed)
• tor-browser-build#12631
• tor-browser-build#32355

Backporting

Timeline

• Immediate: patchset needed as soon as possible
• Next Minor Stable Release: patchset that needs to be verified in nightly before backport
• Eventually: patchset that needs to be verified in alpha before backport
• No Backport (preferred): patchset for the next major stable

(Optional) Justification

• Emergency security update: patchset fixes CVEs, 0-days, etc
• Critical bug-fix: patchset fixes a bug in core-functionality
• Consistency: patchset which would make development easier if it were in both the alpha and release branches; developer tools, build system changes, etc
• Sponsor required: patchset required for sponsor
• Localization: typos and other localization changes that should be also in the release branch
• Other: please explain

Merging

• Merge to mullvad-browser - !fixups to mullvad-browser-specific commits, new features, security backports
• Merge to base-browser -!fixups to base-browser-specific commits, new features to be shared with tor-browser
  • NOTE: if your changeset includes patches to both base-browser and mullvad-browser please clearly label in the change description which commits should be cherry-picked to base-browser after merging

Issue Tracking

• Link resolved issues with appropriate Release Prep issue for changelog generation

Review

Request Reviewer

• Request review from an applications developer depending on modified system:
  • NOTE: if the MR modifies multiple areas, please /cc all the relevant reviewers (since gitlab only allows 1 reviewer)
  • accessibility : henry
  • android : clairehurst, dan
  • build system : boklm
  • extensions : ma1
  • firefox internals (XUL/JS/XPCOM) : ma1
  • fonts : pierov
  • frontend (implementation) : henry
  • frontend (review) : donuts, richard
  • localization : henry, pierov
  • macos : clairehurst, dan
  • nightly builds : boklm
  • rebases/release-prep : dan, ma1, pierov, richard
  • security : ma1
  • signing : boklm, richard
  • updater : pierov
  • misc/other : pierov, richard

Change Description

Note: this is a twin MR of tor-browser!971 (merged).

This MR moves the --disable-eme Firefox configuration option from the base-browser mozconfig to the mozconfigs of targets which support it. At the moment, these are all the desktop targets for which the Mullvad Browser is built. linux-arm (which is not built) and linux-aarch64 (which is a work in progress) do not support --disable-eme, so their mozconfigs must not contain this option. As discussed in issue #282 (closed), when not supported, no --disable-eme option is equivalent to --disable-eme itself (i.e. EMEs are already disabled).

How Tested

grep -RI "browser/config/mozconfigs/base-browser" # discover who uses the base-browser mozconfig
grep -RI "browser/config/mozconfigs/mullvad-browser" # discover who uses the mullvad-browser mozconfig

Then added --disable-eme to every mozconfig that imports base-browser (except for mozconfig-linux-arm, which in any case is not built) and removed it from base-browser itself. I tested this is equivalent to the current configuration by building the Mullvad Browser for linux-x86_64 (./mach build using mozconfig-linux-x86-64-dev) and looking for the relevant entries in about:config.