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.