Try to start windows in safe-mode Windows safe-mode

Firefox opened in safe mode. Once I went back to normal mode it still is not working.

A more general Windows problem, such as issues with the registry file or an incompatible security tool, may be preventing Firefox from opening correctly. If Firefox has launched successfully in the past, use the Windows System Restore tool ( Windows System Restore Tool) to roll back the operating system's configuration to a previous point in time. Ensure you're running the latest versions of your anti-virus and anti-spyware tools and check with the official developer websites for details of any potential conflicts with the Firefox browser.

A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

   https://support.mozilla.org/kb/Server+not+found
   https://support.mozilla.org/kb/Firewalls 

   http://kb.mozillazine.org/Browser_will_not_start_up

Also try:

The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.
Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Did this fix your problems? Please report back to us!

Thank you.

Do a malware check with some malware scanning programs on the Windows computer.
Please scan with all programs because each program detects different malware.
All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender: Home Page:
  http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

Done all that. As I said it works on the guest account but not on mine. It was fine yesterday. Computer is clean.

run the firefox on administrator mode right click on firefox icon enter the selection run as administrator

run the firefox on administrator mode right click on firefox icon enter the selection run as administrator

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test to see if that helps.

Note that using System Restore can cause your Firefox installation to get corrupted because not all files are restored (only files in a white-list), so be cautious with using System Restore.

• You may lose data in the Firefox profile folder like .js and .ini files.
• You may need to (re)install software that was affected.

In case you need to reinstall Firefox then you need to delete the Firefox program folder before reinstalling to make sure that all files get replaced.

Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

Please report back to see if this helped you!

Thank you.

Have reset Firefox but still will not work. On the AVG help part the instructions are out of date, my AVG is 2014. There is no list of applications. I appreciate your help, its driving me crazy lol.

Thanks but I have done all that and makes no difference. It opens under the guest part of the PC, but not mine.

Thanks, but it would not work. :(