Hi Petra, have you tried starting Firefox in its Troubleshoot Mode? To do that, hold down the Shift key while starting Firefox. You should get a little dialog with "Open" and "Refresh" options. Try Open. Will Firefox start and run normally?

For possible future reference, this article describes what the Refresh option does: Refresh Firefox - reset add-ons and settings.

Thank you for your answer. But it is not correct. I did NOT start Firefox in error mode. The start always went (and goes) smoothly. But during the day Firefox keeps crashing and I have to completely restart my PC. That's what happens to me with Google Chrome. But I would like to go back to Firefox if I could work again without any problems - like before. I hope you can help me.