Perform the suggestions mentioned in the following articles:

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

Check and tell if its working.

Not related to your problem but some of your Firefox Plugins are out-dated

• Update All your Firefox Plugins -> https://www.mozilla.org/en-US/plugincheck/
• When Downloading Plugins Update setup files, Remove Checkmark from Downloading other Optional Softwares with your Plugins (e.g. Toolbars, McAfee, Google Chrome, etc.)

See:

• http://kb.mozillazine.org/Corrupt_localstore.rdf

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Problem solved: Since I deactivated FF Fullscreen 7, the situation is normal again. Thank you for your help.