Could you try re-running the Firefox installer? Grab the full installer for your language from this page:

https://www.mozilla.org/firefox/all/

No need to uninstall first. (Better not to.)

Which Firefox locale are you using?

Thanks jscher2000. I have actually uninstalled and re-installed a number of times using the full installer before I posted the question. I have also restarted the PC a number of times after installation, but firefox still did not start. So, I uninstalled firefox after I posted the question and went off to work.

When I received your message, I use the installer I had downloaded previously to install and like magic firefox is opening now! Thanks,

As I said, I have this problem too. The upgrade to v27 was done automatically. Can this be changed so that I will decide weather to upgrade or not? I have the exe file of v26 and after uninstalling v27 and running v26, all was fine until I exited Mozilla: when I tried to restart it, nada.

Hi aniyael, are you able to start Firefox 27 in its Safe Mode? Hold down the Shift key when starting the program. Then in the small dialog that should appear, click "Start in Safe Mode" (not Reset).

Regarding update settings, see: Advanced panel - Accessibility, browsing, network, updates, and other advanced settings in Firefox.

Thanks jschaer2000: after starting it once in safe mode, I closed Mozilla and tried several restarts in normal mode. So far, all worked:-) Thank you very much again. Still the automatic update process bothers me: it took me several days to discover the reason to the malfunction, since when I removed the problematic version from Add and Remove Programs, I didn't pay attention at first that it was another version, not v26 which I installed from my folder. In addition, I like to save the installation files before I run new programs and if I didn't have v26 exe file, I couldn't have operated Mozilla at all. Not that it was fun to remove and install again each time v27 didn't react, but it was better than nothing, I still prefer Mozilla over its competition.

Hi jschaer2000, I was too optimistic: since my last post, I removed Mozilla and reinstalled v26 two times. This morning, it started but 10 minutes after closing it, it stopped responding. I had to remove/reinstall v26. I disabled a few days ago automatic installation of updates, hoping this would do the trick but this morning I was disappointed. How can I continue with v26?

Hi aniyael, changing from "automatically" to "check for updates" has always worked for me. Sometimes a dialog may pop up and if you were in the midst of typing you might accidentally approve the update to run. But otherwise, I don't know why that wouldn't work for you.

Note: Windows system restore can lead to unpredictable results; I don't recommend using that.

I'm not sure why Firefox stops working. Is it possible that some other software is trying to inject itself into the browser? You might want to run some supplemental malware scans. This article lists tools other Firefox users have found helpful: Troubleshoot Firefox issues caused by malware.

Thank you, jschaer2000. Meanwhile, it works. How long, we'll see;-)