Actually yes, I meant Safe Mode for Firefox.
I have some programs open right now... and I haven't restarted the computer yet, but I won't forget next time I reboot... And if it starts fine in Safe Mode, then I will have to find the bad add-on... It is just funny that after trying the first time and killing the process it works fine... Maybe it would also be interesting (as an experiment) to start "Windows" in Safe Mode with networking enabled and see if then Firefox starts fine the first time.
I am really not sure, but I think I had installed the latest version of Firefox when this problem first appeared (and I noticed), then I reinstalled an old version over it, but the problem didn't go. I am not sure because I hibernate a lot, so it took some time before I restarted the computer after installing the latest version...
It was very strange with the latest version. It actually started, but there were no fonts in the tabs and they were not transparent... After killing the process, it worked fine!