Do you have the Firefox icon on your quick launch? Close Firefox. Right click on the icon and select Open New Tab. Sometimes this works. If it opens, close it by Menu > File > Exit.

This is not a cure but will make it easier if Firefox locks up in Windows. https://support.mozilla.org/questions/997866 {web link}

Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Then in the reply box at the bottom of this page, do a right click in the box and select Paste. This will show us your system details. No Personal Information Is Collected.

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

• "3-bar" menu button > Exit (Power button)
• Windows: File > Exit
• Mac: Firefox > Quit Firefox
• Linux: File > Quit

See also:

• http://kb.mozillazine.org/Kill_application

Interesting on Saturday there was an update that fixed my problem with Firefox, but now Thunderbird does not work anymore...

https://support.mozilla.org/en-US/questions/1104485

This problem has returned but WORSE! Now Firefox will not open at all and will open in safe mode ONLY ONCE just like above unless I reboot.

https://support.mozilla.org/en-US/questions/1112941

Is that in Firefox Safe Mode or Windows Safe mode?

If it works in Safe Mode and in normal mode with all extensions (3-bar button/Tools > Add-ons > Extensions) disabled then try to find which extension is causing the problem by enabling one extension at a time until the problem reappears.

Close and restart Firefox after each change via *3-bar button > Exit (Power button)

• Windows: File > Exit
• Mac: Firefox > Quit Firefox
• Linux: File > Quit

You can use the MSConfig program or the Autoruns utility to see what software is getting started (be cautious with disabling services).

• http://technet.microsoft.com/en-us/sysinternals/bb963902.aspx

Do a malware check with several malware scanning programs on the Windows computer.

• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

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:
  http://windows.microsoft.com/en-us/windows/using-defender
• 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

If all else fails, Install an older version of Firefox {web link}

FredMcD said

If all else fails, Install an older version of Firefox {web link}

Personally I would try to avoid at all costs installing an older insecure version of Firefox. If necessary use some other alternative up to date browser.

If for temporary troubleshooting purposes you need to use a different Firefox version consider trying ESR. At least that should be secure, although Mozilla only recommends it for use where it has support from an IT department.

• https://www.mozilla.org/en-US/firefox/organizations/

Additional tip The 2nd part of this is a bit of a long shot

1. Try a new additional profile for test purposes.
   • Use: keyboard shortcut Winkey+R to get the Run dialogue, and enter firefox -P
   • See Profile Manager - Create, remove or switch Firefox profiles
2. Make sue you try with Firefox set to its defaults of NOT clearing History when closing down. (That may have issues in current and recent Firefox versions) {Using Firefox 45 beta may avoid some of those issues )