firefox looks like a mess on linux, it still uses a gtk3 stylesheet for context menus etc. despite everything else being changed with firefox themes (even the window decoration)
i don't know if anyone else submitted this idea before, but either:
otherwise on linux it leads into an extremely inconsistent design, so firefox themes/colour that you can download on the addons website should style everythingor have libadwaita styling for whatever's using a gtk3 stylesheet right now
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Potentially related to your first point:
Themes: allow theming context menu (and other stuff)