A possible cause is security software (firewall,anti-virus) 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 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

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

hello lisayoumans, from another user we found out that this might be related to malware active on your pc. please close all firefox windows and try running firefox in safemode (by pressing the shift key while you're starting it). in firefox safemode go to firefox > addons > extensions and remove the entry named "Codec v1.0" or any other addons that look suspicious & you have not installed intentionally. afterwards see if you can run firefox normally again...

please also do an in-depth scan of your system with the security software already in place and/or a different tool like the free version of malwarebytes & keep firefox and your plugins up-to-date in order to stay safe on the web.

Troubleshoot Firefox issues caused by malware

Update Firefox to the latest release

https://www.mozilla.org/plugincheck/