rugk@thomas it works with GNOME Tweaks when selecting the dark Adwaita theme here. (I guess this is dark GTK mode)
It is unrelated to the Firefox dark theme.
BTW, if you still struggle or want an easy in-browser toggle, I've got you covered with an add-on anyway 😃 😊…
https://addons.mozilla.org/firefox/addon/dark-mode-website-switcher/?src=external-mastodon
All Jonkman Microblog content and data are available under the