Hi, Please check if this happens in [https://support.mozilla.com/en-US/kb/Safe%20Mode Safe Mode.] [http://kb.mozillazine.org/Problematic_extensions Problematic Extensions] [https://support.mozilla.com/en-US/kb/Troubleshooting%20extensions%20and%20themes Troubleshooting Extensions and Themes] [http://support.mozilla.com/en-US/kb/Uninstalling+add-ons Uninstalling Add-ons] [http://kb.mozillazine.org/Uninstalling_toolbars Uninstalling Toolbars]

That was the first thing I tried. It does happen in safe mode too.

Try this : Toggle in FullScreen mode by clicking F11 in keyboard, move the cursor in the up-right area of the window and try to minimize now. thank you