Are you able to start Firefox in its Safe Mode? Hold down the Shift key when starting Firefox. With luck you will get a small dialog.

In the dialog, click "Start in Safe Mode" (not Reset)

Any change?

Thanks for advice, but FF doesn't start with Shift key. Had to reinstall it again to make it work.

After exiting and launching FF for second time firefox.exe appears for 1 or 2 seconds in active processes in TM and dissapears. No errors shown.

Try going to Firefox In Any Language and get the full installer.

Some external process may be tampering with Firefox. Could you run some supplemental malware scans? This article lists tools other Firefox users have found helpful: Troubleshoot Firefox issues caused by malware.

No malware found. It's likely something wrong with app itself. I suppose there are logs, that can clarify the reason.

Hello amaj7, any luck if you boot the computer in Windows Safe mode with network support (press F8 on the boot screen) ?

also, in your system details it seems that you have in your profile folder a User.js file. The user.js file does not exist by default. If you create the file, forget it, if you have not create the file then delete the User.js file(some programs create and write in the file also).

thank you

I am having the same problem. Ever since I updated Firefox to 27.0.1 it will not start once I have shut it down.

I have spend about 5 days on this problem and am getting nowhere fast.

I have built a new profile and also completely removed Firefox and reinstalled it. When I downloaded the Startup script so I didn't need to go through IE to reload Firefox it would reload but not start up. I always need to go to the Mozilla site with IE to download Firefox. It works fine after that, but, as soon as I shut it down it will not restart.

I am running XP SP3 with the latest updates and McAfee with the latest updates. FIREFOX is not my default browser.

No luck with this issue. Had to megrate to Chrome.

Hi tberry7403, could you test whether reinstalling Firefox 26 resolves the issue? I will message you a link for the download.

tberry7403 sent me this information about the problem starting Firefox 27.0.1 after an automatic upgrade from Firefox 26.0:

XML Parsing Error: undefined entity
Location: chrome://mozapps/content/extensions/update.xul
Line Number 14, Column 1:
<wizard id="updateWizard" ...

I'm not sure what causes that error. Are others seeing the same thing?

(Window XP SP3, latest updates. McAfee with latest updates)

The problems with 27 have occurred even after a completely clean install. I uninstalled the version that wasn't working. Removed all Firefox folders I could find. I then install 27 and it will not start.

Leaving that version installed I then go to the Mozilla website via IE and do an upgrade of Firefox. Firefox will automatically start up and work normally. When I then close Firefox and try a restart the process briefly shows up in the "Processes" section of the "Windows Task Manager" window but Firefox never starts.

I have since downloaded 26 and it work just fine.

Guess I'm stuck with that.

I'm not sure everyone in this thread has the same issue, so let's see what others report about specific error messages or other clues to the source of the problem.

Anyone else with this issue
Please start your own thread, and mention this thread when you post your own question. It will also help if you try to post from Firefox 27 whilst it is working, and follow the prompts to include troubleshooting information when you post (use the green button to automate that).

If you wish use this special link as it will be recognised by helpers as relating to this issue and cuts out a couple of steps.

Please say what the Operating System & Version is, and whether it is a keyboard or touchscreen device.

Are you seeing any errror messages, or able to find error logs relating to the problem.

tberry7403

My suggestions: Try reinstalling Firefox 27 again.

This time however leave Firefox 26 for now
(Probably worth replacing that with ESR later if Fx27 still fails)
But do a Custom install of Firefox 27. Choose some other location for the install. Maybe placed adjacent to the standard install and use a folder .../Mozilla-2014/

It is also worth creating a new profile. It is a good idea to use a separate profile for multiple versions of Firefox are in use. It is also worth having disposable profiles as you can then test with them and clear them entirely or otherwise make Firefox start with a new profile. It may be unlikely that profile changes prevent Firefox restarting, but that is something worth testing and ruling out.

• http://kb.mozillazine.org/Testing_pre-release_versions#Installing_multiple_versions