Does Firefox not start in Safe Mode?

• Click the Windows logo on the taskbar to bring up the Start Menu.
• In the search box, type firefox.exe -safe-mode and press Enter.
• If the little Firefox Safe Mode window pops up, click the Start in Safe Mode button.

• For more details, see Diagnose Firefox issues using Troubleshoot Mode

If Firefox doesn't start in Safe Mode, does the profile manager start? If yes, create a new profile and try starting Firefox in that profile.

Use the Profile Manager to create and remove Firefox profiles

Also see:

Firefox won't start - find solutions

Firefox does not work - Common fixes to get you back up and running

It is possible that your security software (firewall, anti-virus) 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

• https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating