hello Rokli, this sounds like an issue with adware on your pc. please perform all these steps:

1. reset firefox (this will keep your bookmarks and passwords)
2. afterwards go to the firefox menu ≡ > addons > extensions and in case there are still extensions listed there, disable them.
3. finally run a full scan of your system with different security tools like the free version of malwarebytes and adwcleaner to make sure that adware isn't present in other places of your system as well.

Troubleshoot Firefox issues caused by malware

Hello Philip, Thanks for your quick reply. Unfortunately I cannot open Firefox and consequently I can neither do a reset nor go to the menu and disable extensions. The screen turns off as said before. I can go the internet only by using internet explorer.

have you tried to reinstall the latest version (33.1.1?) from https://www.mozilla.org/firefox/all/?

otherwise you can also press the shift key while launching the browser and reset from the dialog that appears then or continue to launch firefox in safemode from where you can remove addons or disable hardware acceleration which also might cause display issues in the firefox menu ≡ > options > advanced > general