-> Start Firefox in Safe Mode to check if one of the extensions 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.com/kb/Safe+Mode 

-> click the Firefox button, go to History menu and click Clear Recent History... -> Time range to clear: select "EVERYTHING" -> click Details (small arrow) button -> place Checkmarks on Cookies, Cache -> click Clear Now

-> Update ALL your Firefox Plug-ins https://www.mozilla.com/en-US/plugincheck/

-> Search for faulty extensions

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

-> Also check for "Corrupt extension files"

  http://kb.mozillazine.org/Unable_to_install_themes_or_extensions 

Check and tell if its working.

Make sure that you do not run Firefox in permanent Private Browsing mode.

• https://support.mozilla.com/kb/Private+Browsing
• You enter Private Browsing mode if you select: Firefox > Preferences > Privacy > History: Firefox will: "Never Remember History"
• To see all History and Cookie settings, choose: Firefox > Preferences > Privacy, choose the setting Firefox will: Use custom settings for history
• Deselect: [ ] "Permanent Private Browsing mode"