A possible cause is security software (firewall) 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 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.com/kb/Server+not+found
• https://support.mozilla.com/kb/Firewalls

Thanks for your tip. I don't know if I have a firewall and even if I do, I don't know how to find permissions. So I'm not able to do this. Any other advice?

Just to be sure that I understand what is happening.

You do not get a browser window if you try to start Firefox or do you get at least something?

See:

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