Read all your last issues.

Did not see anyone point you to the Full Version Installer for 56.0.1 https://www.mozilla.org/firefox/all/

Try uninstall, yank out all the Mozilla Firefox Folders except your profile folder , run a registry cleaner if have one then install Full Version.

You may want to test your ram as it could be corrupt. https://www.memtest86.com/

Please let us know if this solved your issue or if need further assistance.

Thank you! FF 56 appears to be working fine now.

Looks like the full version installer was what was needed.

I didn't know there is a difference in kinds of installer. I suppose the full installer overwrites all the Firefox files and components, while the others don't?

And now the problem is back. When it was working I installed a video downloader plug, after which it went back to the same startup failure. Now, even uninstalling, running CCleaner and disk cleanup and going back to the full version installer doesn't help.

Should I eliminate all firefox files in the appdata and try again?

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

• https://support.mozilla.org/en-US/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 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

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10