Go to your Start Menu or Button, click Mozilla Firefox (safe mode) click restore original firefox settings and restart session.
There is no folder for Mozilla Firefox in the Start menu. Weird. It’s on my own home PC, I know that. When I go to Start, the only thing Firefox related is just “Mozilla Firefox” but when you clicked on it, it launches Firefox only. There is no folder. It’s basically just a link to launch the regular mode.
I managed to try that by holding down the SHIFT key while launching firefox. I put a tick mark in the box you said, but I still have google for the address bar.