Does Firefox works or you get an Firefox error 404?

Error 404 is an error for a wrong link / dead link. If Firefox open and the first page is error 404 just change your startpage at settings.

404 is page not found. Is Firefox connecting to anything? Have you checked your firewall?

• http://kb.mozillazine.org/Firewalls
• https://support.mozilla.org/kb/Firewalls

Some problems occurs when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version)

• Remove Firefox from your program's list of trusted or recognized programs, then add it back. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}

You can check the target line in the Firefox desktop shortcut (right-click: Properties) to make sure that nothing is appended after the path to the Firefox program.

Can you start Firefox in Safe Mode by holding down the Shift/Options key?

Try to start Firefox with the about:blank page by creating a copy of the current Firefox desktop shortcut and add a space and -url "about:blank" to the command line.