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Hi, try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem. Thank you!

Everything is fine now, thank you very much!!

Hi, thank you for sharing your extensions list (next to your question: Question Details > More System Details). Your Classic Theme Restorer is listed as the November 18, 2016 version so it probably needs to be updated to resolve this issue.

You can trigger an update check on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then look for a small gear/wheel/cog icon above the list, click that button, and "Check for Updates."

That should work for extensions installed directly from the Mozilla Add-ons site, but locally installed extensions may not update. In that case, you can install again to "update" the currently installed version (don't remove what you have, to avoid possibly losing settings).

https://addons.mozilla.org/firefox/addon/classicthemerestorer/