Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window. You have to close and restart Firefox after each change via "File > Exit" (on Mac: "Firefox > Quit")

Also check the Startup and Home page setting. Firefox > Preferences > General > Startup

This is the only thing that worked for me:

Go to Tools, Options, General. I have 'Show my home page'. I noticed that on my Home Page there was 'about/ about/ about' (one for each untitled page that kept popping up). I deleted everything before the 'http', clicked OK and Tah dah.......it worked!

Hope this helps others who now have no hair due to pulling it out!

Thanks for the tip! Much appreciated! It worked spot-on after I struggled most of the day trying to fix it!