Start '''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Firefox in Safe Mode]''' {web Link} by holding down the '''<Shift><br> ''(Mac Options)'' ''' key, and then starting Firefox. Is the problem still there?

Yes. It is also happening on another computer running Win 8.1 Pro. Thank you

'''[https://www.mozilla.org/en-US/plugincheck/ Use this link to check that your plugins are up-to-date.]''' If any are listed as unknown or research, do not worry about them. Start your '''Computer''' in safe mode with networking. Then start Firefox. Try '''Safe''' web sites. Is the problem still there? '''[http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting The Computer In Safe Mode;<br>Free Online Encyclopedia]'''