Was that only a one off problem or are you still having that problem.

If you still have a problem first of all try Firefox's safe mode, then if necessary do a Refresh, but make a note of any Firefox extensions. I suggest bookmarking their home pages.

• Diagnose Firefox issues using Troubleshoot Mode
• Refresh Firefox - reset add-ons and settings

Off topic but presumably you realise you are running short of time if you wish to take the free upgrade from Windows 8.1 to 10

• https://support.microsoft.com/en-us/help/15356/windows-8-install-update-kb-2919355

I believe the offer ends at the end of this month. It does give an option to downgrade again if you do not like it, but again that must be done within time limits.

The problem appared one or two times, i don't remember. I don't have any plugins. I would like to know: perhaps there is a limit to maximum cache size?

Cache can be good it may help speed up Firefox. Sorry I am not aware of any limits Firefox imposes on Cache Size, but that is not saying there are no limits.

However cache may increase when there is a problem, and large amounts of cache on a computer with nor much available RAM will cause slowdowns. If you have only seen this a few times it is probably not worth speculating.

Note you may check what is in the cache using about:cache <- Key into the address bar and press Enter

Also see

• Firefox uses too much memory or CPU resources - How to fix
• Firefox uses too much memory or CPU resources - How to fix
• How to clear the Firefox cache
• Firefox hangs or is not responding - How to fix