see screenshot, this should fix your issue, you can find that setting under the ff general settings tab. tools/options/general/ scroll all the way down..cheers

Now smooths as never before thanks! Crazy a normal user need to know all these settings (For other reading: the setting to tweak is "content process limit:1" while "use hardware acceleration" can be enabled) Hope next firefox updates will not change that option or I will forgot to change back! Thanks again Regards

anytime mate..cheers

CRAZY update: reverted the "content process limit" option (for testing) to default 8 and now fine the same! (Was quite easy to repeat the problem it just needed something like 10 tabs opened, tabs with flash content like youtube, facebook, and then close them). I will keep continue monitoring the situation, for sure changing that option made the trick in somehow!

My system has 12 GB RAM but slow CPU (AMD Phenom II X2) and the problem I had was with the CPU being stucked soon irremediably

edit: the problem came back the next day, now I set the option to 1 or 2 (that is the number of my CPU cores) and perform a deep test

You can check the Firefox Task Manager (about:performance) and about:memory pages.

• https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox
• https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-or-cpu-resources

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> General: Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting for changes to take effect.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

Sorry squirell if have to undo the best answer but hope helps others. Today the problem* occurred whatever "Perfomance" configuration so must be elsewhere and thanks to cor-el realized was existing a "firefox task manager" that easily showed the tabs/extensions "Energy Impact" (indeed from windows task manager is not possible to understand that)

https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox

In my specific case the issue was with the add-on "IDM Integration Module" (Download files with Internet Download Manager that try to catch all flash/video contents of the page, maybe have a bug something like CPU/memory leak)

• Once 80% of CPU was granted by Firefox process there was no way to release even closing tabs

see screenshot, this should fix your issue, you can find that setting under the ff general settings tab. tools/options/general/ scroll all the way down..cheers

This fixed mine. I was getting constant lock up and freezes in Firefox. I had to reboot the computer to get it back. I gave up on FF and moved to Chrome.