What computer system and Firefox are you using?

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

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Use "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: Firefox > Quit Firefox; Linux: Firefox/File > Quit) to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

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

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

Do a malware check with several 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:
  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
• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

The solution that is described in https://support.mozilla.org/questions/997866 doesn't work at all. It shows "SUCCESS: The process with PID 5912 has been terminated", but the process is still running. If it was possible to kill the process, I would just kill it using Task Manager.

No solutions, which you had provided, worked for me. I cannot start Firefox. It shows the message "Firefox is already running, but is not responding. The old Firefox process must be closed to open a new window.". After I close the message another message is shown: "Your Firefox profile cannot be loaded. It may be missing or inaccessible.". I can start Firefox with a new profile, but I still cannot open any page.

asdqweasd said

The solution that is described in https://support.mozilla.org/questions/997866 doesn't work at all.

I am sorry. I have not heard of it failing. It uses a Windows program to shut Firefox down.

Have you tried Firefox in Safe Mode?

I have finally finished scanning for malware on mo computer. Nothing was detected except some spying cookies.

I have started Firefox in Safe Mode, but the it hanged after about a day of running, so the problem remains.

There is another discussion of this issue, so multiple users have the same problem, which prevents using of Firefox normally. http://superuser.com/questions/884996/windows-7-wont-close-programs-even-in-task-manager

asdqweasd said

There is another discussion of this issue, so multiple users have the same problem, which prevents using of Firefox normally.

I read it. Very odd. The Task Manager should be able to shut down anything.

I just had a chat with a Microsoft tech; https://pastebin.mozilla.org/8826478