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

Firewall is not the case. I made a backup for the profile and edited the profile config file, it worked, but after closing firefox and trying to turn it on again it broke. I don't know what to do to make it work without any trouble :P That is really frustrating

Can you please shed more light on this line?

   "There seems to be a problem with some setting files. When i moved them elsewhere, it worked"

I didn't quite understand it. What were the files that you moved? Where did you move them from? Where did you move them to?

Hi borek555, you wrote:

I made a backup for the profile and edited the profile config file, it worked, but after closing firefox and trying to turn it on again it broke.

Which file did you edit and what did you do to it?

If you have software like Advanced SystemCare that might reset some files to older versions to protect these files against changes then check the settings or uninstall this software.

I was doing backups of the profile folder and trying to edit the profiles.ini file. Well, I was doing that a bit chaotically in desperation (in different order) and sometimes firefox started to work, but after turning off the firefox window, the problem started to come back. And, Avast doesn't seem to affect that (even when i turned off the protections the problem still occured)

When Firefox won't start normally, does it start up either in Safe Mode or in the profile manager?

To start Firefox in its Safe Mode, with firefox.exe not running, hold down the Shift key when launching Firefox.

To start up the Profile Manager, with firefox.exe not running, type or paste the following in the Start menu search box and press Enter:

firefox.exe -P

The list of existing profiles in the profile manager should match what you saw in profiles.ini.

I'm not sure what Firefox would be saving at first shutdown that might cause it not to be able to start up again, or whether any external process could be altering your new profile during or after its first run.

You might try a comparison of two new profiles: create one and don't run it, then create a second one and start up in that one, then exit and confirm that it will not start again. Then compare the two profile folders. For file and directory comparisons, I usually use WinMerge: http://winmerge.org/.