I just noticed it yesterday evening: plasma-integration for Firefox browser is not working again as FF extension at least. And if you re-install it FF offers you just an obsolete kde plasma integration extension to retry again.
Well even though this discussion is many years old (2018). It's still doing today, with Firefox 125.0.1 in my case ( check pic here https://i.imgur.com/zorAESi.png ) I hope some one could help pls.
Not working plasma integration extension and FF browser
Check upstream bug reports to see if you can find a similar issue:
https://bugs.kde.org/buglist.cgi?list_id=2692313&product=plasma-browser-integration&resolution=---
demm Thanks. It's been already reported and still not solved , affecting some FF and derivatives like Librewolf and others. This is not happening in EOS KDE with FF and Fedora KDE for myself checked it up yesterday. https://bugs.kde.org/show_bug.cgi?id=476967