Firefox 141 does not honor Cinnamon's dark mode on Mint 22.1

Workaround

For Mint 22.1:

dconf write /org/gnome/desktop/interface/color-scheme "'prefer-dark'"

source: https://bugzilla.mozilla.org/show_bug.cgi?id=1978639#c24

If you need to automatically switch between light and dark mode, see this more advanced workaround: https://github.com/linuxmint/cinnamon/issues/12994#issuecomment-3190398183

For Mint 22.2:

This issue should be fixed. If you still have this problem, please see this comment: https://github.com/linuxmint/cinnamon/issues/12994#issuecomment-3249853445 and the comments following it untill this one: https://github.com/linuxmint/cinnamon/issues/12994#issuecomment-3252938783

Update: fix is coming (now released for Mint 22.2)

A fix is in the pipeline for Linux Mint.
See: https://github.com/linuxmint/cinnamon/issues/12994#issuecomment-3137278378

Original bug report

Distribution

Mint 22.1

Package version

6.4.8

Graphics hardware in use

AMD Radeon RX 9070 XT

Frequency

Always

Bug description

Full details: https://bugzilla.mozilla.org/show_bug.cgi?id=1978639

Last few comments are most relevant, specifically comment #28:

Fairly sure now the issue is your distro is missing https://github.com/flatpak/xdg-desktop-portal/pull/1538 and related changes (not sure what needs to happen where to get all those fixes into linux mint).

So basically before that change (https://github.com/flatpak/xdg-desktop-portal/commit/49ee1c0d535608f6a5d0a47a1cd510958dc5e34f in particular) you could get different (conflicting) settings from two different portal implementations.

It might be that cinnamon wants to set the /org/gnome/desktop/interface/color-scheme regardless tho, since that'd trivially "fix" this issue?

Steps to reproduce

1. Upgrade Firefox to version 141.0+linuxmint1+xia using Update Manager
2. Set Firefox's color theme to "System theme — auto"
3. Set Cinnamon to a dark theme
4. Close all windows of Firefox
5. Launch Firefox

Expected behavior

Firefox's UI should be in dark mode

Additional information

I'm not sure if this is relevant, but as my GPU is very recent I am using a newer Linux kernel and a newer version of Mesa than those that Linux Mint offers by default. This was done according to the instructions here: https://forums.linuxmint.com/viewtopic.php?t=424779

Please see the linked Bugzilla for all relevant info: https://bugzilla.mozilla.org/show_bug.cgi?id=1978639

Start with the linked Bugzilla report and the referenced Cinnamon comments about xdg-desktop-portal and Mint 22.2. Reproduce the Firefox dark-mode behavior on the stated Mint versions and confirm that Cinnamon's fix makes Firefox use dark mode without the workaround.