Hello, open the Firefox Profile Manager! How to do this: press Windows Key + R on your keyboard to open up the run box then type in "C:\Program Files\Mozilla Firefox\firefox.exe" -P and create a new profile -- Firefox should then open as normal.

Thanks very much Mkll, that worked and can now open Firefox again. However, very unhappy with Quantum, extremely clunky and slow so probably going to give Chrome a go. Real shame, been really happy with Firefox for years, did everything I wanted it to do and quickly. Note to Mozilla, don't try and fix what ain't broken!