hello davidcb13, please close all firefox windows & try launching firefox in safe mode by pressing the shift-key while/after you start the program. in case you can open firefox this way, continue in safemode & either try to remove any extensions or themes that might cause the issue in firefox > addons. in case this doesn't work you can alos try disabling hardware acceleration in firefox > options > advanced > general.

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

Thank you very much! Turning off hardware acceleration did the trick. Would have never known/thought about that option.