It is a different system than the one you posted from???

In the absence of crash reports, let me mention something that is "going around" in case you have this problem:

Firefox is crashing within seconds of startup if you have either of the Applian Technologies programs Freecorder or Replay Media Catcher in combination with Norton security software (and possibly some other security software).

If you have either of those Applian programs and it isn't essential, try uninstalling it and then restarting Windows to ensure that none of its components are left running.

Will Firefox start normally after that?

If you prefer not to uninstall the program, a few other users said using the msconfig program to prevent FlvSrvc.exe from running at startup bypassed the problem. See: http://windows.microsoft.com/en-us/windows/using-system-configuration

Yes - I had to send the question from my laptop; it's FF on my PC that has the problem (but as I can't get into it, I can't even tell you what version it is!) I have none of those 'problem' programs you mention, nor does flvsrvc run at startup. So, is the solution to uninstall FF and then reinstall? I will also run my AV prog (windows defender) just to make sure the problem website isn't infected somehow. Is there any way of accessing the data in 'about:config' without going via FF itself? Is that file an XML file?

starman said

So, is the solution to uninstall FF and then reinstall? I will also run my AV prog (windows defender) just to make sure the problem website isn't infected somehow.

That usually doesn't help. I'll suggest other things in a moment.

Is there any way of accessing the data in 'about:config' without going via FF itself? Is that file an XML file?

Most of the about:config preferences but not all are stored in prefs.js in your profile folder. This article describes how to find your profile folder when Firefox is not running: Profiles - Where Firefox stores your bookmarks, passwords and other user data.

But what I suggest is the following:

(1) Safe Mode Test

Try starting Firefox in its Safe Mode. That's a standard diagnostic tool to deactivate extensions, hardware acceleration, and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any less crashy?

(2) Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent, corrupted, or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(A) Download a fresh installer for Firefox 41.0.1 from https://www.mozilla.org/firefox/all/ to a convenient location. (Scroll down to your preferred language.)

(B) Exit out of Firefox (if applicable).

(C) Rename the current program folder, either:

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\OldFirefox

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\OldFirefox

(D) Run the installer you downloaded in (A). It should automatically connect to your existing settings.

Will Firefox start normally or in Safe Mode after that?

Note: Some plugins may exist only in that OldFirefox folder. If something essential is missing, look in these folders:

• \OldFirefox\Plugins
• \OldFirefox\browser\plugins

Otherwise, there's no reason to keep the old folder except research/curiosity.

Sorry, I ignored the part about the problem website.

Do you mean something changed your home page, or the problem is Firefox trying to restore a crashed session?

You also could check your Firefox desktop shortcut to see whether it is hacked with the unwanted address. Either:

• Desktop shortcut: right-click the icon, choose Properties
• Pinned taskbar icon: right-click the icon, right-click Mozilla Firefox, choose Properties

Windows normally will select the Shortcut tab. If not, go ahead and click the Shortcut tab. The Target line should not have anything after this part:

• 64-bit Windows: "C:\Program Files (x86)\Mozilla Firefox\firefox.exe"
• 32-bit Windows: "C:\Program Files\Mozilla Firefox\firefox.exe"

Thanks very much for your replies, but problem solved! Yes, FF was trying to restore that 'bad' website and I suspect the cause may have been a cookie or PHP session which has now lapsed. Everything working properly again, but once more, many thanks for your help.

Maybe I spoke too soon! That particular problem went away after I removed the 'restorable' pages (one of which was the problem page) when FF restarted. But after that, the screen changed (i.e., white menu items on a pale BG etc), FF would not launch from the desktop shortcut, and it could not be removed through 'add/remove programs'. Therefore I removed the FF folder (the best I could do at the time) and downloaded version 35 which seems to be working; it's what I am using now. But all possible digits are crossed...

Firefox 35 has a lot of known security vulnerabilities. If you think this is a problem with Firefox 41, you could consider using a different variant of Firefox called the Extended Support Release (ESR) for the time being. ESR is designed for companies that require a longer cycle time between feature changes, but anyone can run it.

ESR currently is based on Firefox 38. It will get security updates over the next several months, but not get new features, so any issues introduced by feature/function changes in Firefox 39, 40, and 41 will not kick in for quite a while.

If you decide to try it, here is how I suggest installing it:

Clean Reinstall

We use this name, but it's not about removing your settings, it's about making sure the program files are clean (no inconsistent, corrupted, or alien code files). As described below, this process does not disturb your existing settings. Do NOT uninstall Firefox, that's not needed.

(A) Download a fresh installer for Firefox 38.3.0esr from https://www.mozilla.org/firefox/organizations/all/ to a convenient location. (Scroll down to your preferred language.)

(B) Exit out of Firefox (if applicable).

(C) Rename the program folder, either:

(32-bit Windows folder names)

C:\Program Files\Mozilla Firefox

to

C:\Program Files\Fx35

(64-bit Windows folder names)

C:\Program Files (x86)\Mozilla Firefox

to

C:\Program Files (x86)\Fx35

(D) Run the installer you downloaded in (A). It should automatically connect to your existing settings.

Success?

Note: Some plugins may exist only in that old folder. If something essential is missing, look in these folders:

• \Fx35\Plugins
• \Fx35\browser\plugins