I had this problem and firefox eventually worked after uninstalling OpenText (formerly Hummingbird) NFS and Exceed. I think they may have some mozilla plugin included though I've not looked into it in much more detail. Appreciate that's a pretty vague solution but thought it may help...

Unfortunately I don't have any of the above plug-ins installed. If only I was provided with a safe mode for FF4 when it says it has installed I would be able to validate more, but alas!

I too have this problem. When this situation occurs, I have to start Firefox in safe mode and disable all addons. After that, FF will launch and then I can activate all the disabled addons. I have noticed that when I activate the IE Tab plugin or the Foxy Proxy Plus plugin, the next time FF won't launch again. However, I redo the sequence (safe mode + disable addons + launch + enable addons) and then everything starts to work as supposed to.

Just like to say I'm still having this problem.

It's so frustrating! I want to upgrade to the latest and greatest but nothing works!

I've even fully repaired my entire Windows partition (it had some slight issues) and this still hasn't worked. Upgraded to service pack 1, still nothing.

Same type of error as well. Installs fine, then when I go to run it nothing happens. The process is running in memory, but no window displays. No add ons are killing it.

Someone help please! Come on Mozilla, you're supposed to be the best... please don't make me go to Chrome... I just don't want to but I also don't want to run broken and/or out of date software!

I have same problem with firefox 4.0 ore 4.0.1 I dont know why, I have to install again firefox 3.0 and i'ts working again. Firevox 4.0 wont work :( I hope than Mozilla teem will fix this problem that it shows in Windows 7 Profesional.

Sounds like the same issue I'm having. On my side it started with FF 3.x yesterday. Updated to FF4 last night. At the install process, if I launch FF4, it'll run, but once that's closed.. gg.

Just tried FF4.0.1, still no joy.

C'mon Mozilla, fix this please! It's been months now that I can't use FF4!

Solved it!

It was a dodgy version of Firebug (1.6.2) running, which would kill the disable/update process. It was killing something on the first run that would corrupt my Firefox.

How I fixed it:

• Download the latest Firefox
• Install Firefox 4 (as Standard) in-place upgrade
• Before finishing, un-tick the "Run Firefox Now" button
• Hit Finish
• Find Firefox in the Start Menu and hold Shift and click the executable (safe mode)
• Leave everything disabled
• You should now have a working Firefox!

Remove/update your add-ons as needed. I only had to upgrade Firebug in safe mode to get it working.

Odd how the check for updates completely broke this Firefox, wonder what was so bad about Firebug.

Hope this helps at least one other person! The "Run in Safe Mode" option is so hard to discover now when your browser is broken!

Damn, this tool doesn't allow you to solve your own problems! Ouch!

I do not have Firebug installed, but I'm very sure that the problem, in my case, is with the FoxyProxy addon. If I disable it, FF will start normally, otherwise, only in safe mode.

Try the following, and it may help you. I was not lucky after uninstalling and reinstalling the program several times, I even manually removed the Mozilla Folder under the User profile which is generally located at C:\Users\username\AppData\Local - yet it wouldn't still work. Here is what I did. I reinstalled the Program, and when you get to the point where you choose the location of the installation, I modified it to something else. By default; the program wants to get it installed at C:\Program Files (x86)\Mozilla Firefox in a Windows 7 64 bits. I changed that to C:\Program Files (x86)\Mozilla Firefoxx (see the extra x) and it worked fine after that. This indicates that some plug ins were stopping the program to work. Unfortunately, I could not tell what plug in was, but I got working that way.

Hope that helps.

Just to be clear, my original problem has been solved.

It was a bad plug-in. I gave the steps for debugging/fixing in my post above.

I can't mark this as solved because the forum software here won't allow my own posts to be the solution.

Hope my above comment help others. (Just remember to prevent the first run and then run in safe mode and you'll see it's all good!)

geezus.. you have to be a frkn programmer to figure this stuff out. Def not user friendly software unless you know a lot about computers. This worked for me too:

How I fixed it:

◦ Download the latest Firefox .

◦ Install Firefox 4 (as Standard) in-place upgrade .

◦ Before finishing, un-tick the "Run Firefox Now" button .

◦ Hit Finish .

◦ Find Firefox in the Start Menu and hold Shift and click the executable (safe mode) .

◦ Leave everything disabled .

◦ You should now have a working Firefox! .

Thanks anntiles - it works. Installed to C:\Program Files (x86)\Mozilla Firefox4 rather than allow it to install to the default C:\Program Files (x86)\Mozilla Firefox

The problem is that your Intel(R) Core(TM) i7 CPU X980 CPU has more than 4 cores. This is a problem that is rare, but not unique to Firefox and not unique to Intel i7s. The corresponding AMD 6 core processors fail the same way. It is not peculiar to Windows 7, or 64 bit operations either. It fails the same way in WinXP 32 bits, for example. It probably fails in Windows 2000. running on a machine with > 4 cores.

The solution is to edit BOOT.INI per http://support.microsoft.com/kb/289022. The option you want to change is called /numproc. You can do this via a GUI using MSCONFIG. Set /numproc=4 and the problem will go away. This is a reversible operation, just requiring a reboot to effect a change.