Problem installing extensions

I have an odd problem installing Firefox extensions - currently on Manjaro Linux, but it is the same on other distributions (e.g. Mageia). The problem has been with me for quite a while now but beinng more of a nuisance I am only just getting around to trying to sort it out. So… The problem…

I try to add a Mozilla approved extension and I get a pop-up with “Cancel” and “Add” buttons. Clicking on either has no effect other than making the button briefly a darker gray. The pop-up won’t go away until I either shut the tab or switch to another tab and then back again.

This happens even if I remove all of the ~/.mozilla tree and start from scratch, but it only happens with my account. So by trial and error I have established that I can make the problem go away by moving my .config out of the way and letting the system to create a new one. Trouble is, that .config tree contains a lot of other stuff which I don’t want to lose. I’ve looked at files and directories within it which have an obvious relation to firefox and mozilla, but to no effect.

Any suggestions what the problem might be and how to cure it without zapping the whole of .config? (Or going through the tedious process of doing a binary chop on that tree?)

Oh, well… I’ve now spent half a day on this and the answer turns out to be: remove .config/kwinrc

Mine accumulated a lot of crap over years and just letting the system re-create it cured the problem, without any noticeable side-effects.