Firefox theming issues in KDE

[Bestia]

Are you trying it under Wayland. If yes then this other user posted about the issue:

Gtk common themes doesn't correctly apply breeze dark under wayland in firefox on kubuntu

I’ve noticed something interesting about running Firefox on Kubuntu: whenever Firefox is run under Wayland, it is able to access the Breeze Light GTK theme just fine from gtk-common-themes, but not Breeze Dark. Instead it falls back to Adwaita-Dark. Interestingly, it is able to access Breeze Dark whenever Firefox is instead run under X11 or XWayland. I wonder why this is…? I did also report this to Mozilla’s bug tracker, but was hoping it would be useful if the snap people could also have a lo…

Although I notice you mention Breeze Black and there isn’t that one in the gtk-common-themes. There are only Breeze and Breeze Dark.