Found a few of these in the event viewer

Faulting application name: firefox.exe, version: 55.0.3.6445, time stamp: 0x599ed8ae Faulting module name: ntdll.dll, version: 10.0.15063.447, time stamp: 0xa329d3a8 Exception code: 0xc0000005 Fault offset: 0x000000000001f8cc Faulting process ID: 0x2248 Faulting application start time: 0x01d3210e269e1954 Faulting application path: C:\Program Files\Mozilla Firefox\firefox.exe Faulting module path: C:\WINDOWS\SYSTEM32\ntdll.dll Report ID: 3e346c1a-e325-4182-9216-1085af725222 Faulting package full name: Faulting package-relative application ID:

Note: V55 has been giving users problems. Reinstall v54.

https://support.mozilla.org/en-US/kb/install-older-version-of-firefox?cache=no Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements

Thanks Fred. Installed v54 as you mentioned and it worked. Afterwards I let firefox update to v55. So far its working like a charm, and the old performance issues I was also seem to be gone. I did lose a few settings that I cannot seem to remember but I will post those in a separate questions. Many thanks for your help! Rui

Glad to help. Safe Surfing.

I hate to re-open this but I realize that the installed version is 32-bit now. If I try any of the above work-arounds with the 64bit (which I had previously running onthis machine) it just doesnt work. In short, any of the 64-bit versions (I have tried 54, 55, and the 57beta) install but the program does not start. I am staying with 32bit for the time being but I would really to be able to fix this and make use of the 64-bit. Thanks in advance

Hello again. It would be best to ask a new question; "32-bit works but 64-bit has problems" Or something like that.

Hi Fred, I managed to get it working late last night by renaming the firefox.exe file to firefox1.exe. I confirmed the issue by renaming it back to firefox.exe which makes it stop working. As long as the 64-bit installation holds the original .exe name it will not work. Even with a work-around I would still like to get rid of the error. So, I will follow your advice and post this as a new clean question.

PS - Though 64-bit version is working with work-around, windows still does not recognize firefox as an installed browser because it keeps asking if I want to make it default to which I have clicking "Yes" to no effect. In the default programs panel, firefox does not show as one of the options for default browser, or is that a different issue? Smells like a registry thing?

Okay so I have to help before I can start a new thread and this is the issue I want to start a new thread about because I have exactly the same problem. I agree with rpaulino that this looks like a registry problem, but exactly what evades me!

On one PC Firefox updated and seemed to hang. After quite some time, I rebooted and Firefox would not start. I've tried all of the measures in the fix firefox crashing list but nothing works. On Windows 10 on an i7 Dell PC with 6Gbytes of RAM, a loading blue circle starts and then nothing happens.

Watching with Task Manager, nothing ever seems to get to start enough to show.

I've uninstalled and cleaned the registry with ccleaner (not a compromised version) and reinstalled and it will not load. Even an earlier version does not work.

I have not tried the 32bit version, but guess like rpaulino it would work.

This one totally evades me as to where to even look for a solution now.