Just to make sure: You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no Run most or all of the listed free to use malware scanners. Each works differently. If one program misses something, another may pick it up.

Many site issues can be caused by corrupt cookies or cache&#46; '''Warning ! ! ''' This will log you out of sites you're logged in to&#46; You may also lose any settings for that website&#46; * [https://support&#46;mozilla&#46;org/en-US/kb/delete-cookies-remove-info-websites-stored Delete cookies to remove the information websites have stored on your computer] {web link} * [https://support&#46;mozilla&#46;org/en-US/kb/how-clear-firefox-cache How to clear the Firefox cache] {web link} If there is still a problem, '''[https://support&#46;mozilla&#46;org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Start Firefox in Safe Mode]''' {web link} by holding down the '''<Shift> ''(Mac=Options)''''' key, and then starting Firefox&#46; A small dialog should appear&#46; Click '''Start In Safe Mode''' (not Refresh)&#46; Did this help? While you are in safe mode; Try disabling graphics hardware acceleration in Firefox&#46; Since this feature was added to Firefox it has gradually improved but there are still a few glitches&#46; ''[https://support&#46;mozilla&#46;org/en-US/kb/forum-response-disable-hardware-acceleration How to disable Hardware Acceleration]'' {web link}