hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Thanks Phillipp. When in safemode there are no problems.

so it may be a conflict between the update and one of your extensions - you could disable them one-by-one (a restart might be necessary after each step) until you find out which one is causing it. then please look on its homepage, maybe there will aready be an updated version available or else contact the extension's developers to make them aware of the issue. thank you

It was Instantfox 2.8.2. Thankfully it was the second one I tried.

Thanks for your help.

haha, sometimes you just get lucky :-)
and maybe you're not of luck yet, since there already seems to be fixed version 2.8.4: https://addons.mozilla.org/en-US/firefox/addon/instantfox/versions/