Comment 116 for bug 220263

(In reply to David Baron :dbaron: ⌚UTC+2 from comment #45)
> The number of comments recently makes me suspect that something got *worse*
> recently.

Yes, something did change in GNOME recently. Prior to GNOME 3.28 there was a GTK preference called gtk-application-prefer-dark-theme that could be used to get dark widgets. This could either be set in ~/.config/gnome-terminal/gtk-3.0/settings.ini. This was also exposed as a toggle option in the gnome-tweaks tool, which is probably how most users had set it it. This GTK option has been deprecated in GNOME 3.28, and the new way to get a dark theme is to explicitly set one's GTK theme to Adwaita dark. You can read about this a bit more at [1] and [2].

Firefox apparently has some logic to ignore gtk-application-prefer-dark-theme, but that logic no longer works on GNOME 3.28 with the Adwaita dark theme. As was mentioned in comment #43, the work around for GNOME 3.28 is to set a user pref (i.e. in about:config or user.js) that sets widget.content.gtk-theme-override to Adwaita:light.

As I suspect the overwhelming majority of Firefox users with dark GTK themes are using Adwaita dark (which is bundled with GNOME), I would suggest updating the Firefox dark them detection logic to handle the new case with the Adwait theme names hard coded. This is hacky, but probably mitigates the issue for most users.

[1] https://jeremy.bicha.net/2017/08/29/gnome-tweaks-3-25-91/
[2] https://jeremy.bicha.net/2018/01/29/gnome-tweaks-3-28-progress-report-1/