I hate when firefox starts downloading updates in the background without telling me. That slows down firefox to a crawl too, but I still have full use of my mouse and keyboard, unlike when firefox goes into a “loop” and everything slows down.
Maybe firefox is doing this only for certain websites. Like facebook does to it’s conservative users.
Check your FF settings to see if the updates are ‘automatic’ instead of manual.
That may be the problem.
It tries to update while in use...................