Because ice-ssb is abandoned on Linux, I searched for alternatives and I found it! This extension works and it feels much better and modern then ice-ssb ever was.
Installation was maybe more complicated then any other add-on, but it was very well guided, so the whole process went without problem.
A very nice suprise is the access to Firefox extensions, settings, about:confing and others from the titlebar. I have never saw a titlebar in Plasma KDE with menus and additional elements! This rocks!