Running Windows 7, all versions of Firefox beyond 3.6.0 will not access websites, showing Error 404 and displaying only the portion of the URL that follows the domain name
Choosing any of several dozen bookmarks, Firefox will show a screen reporting Error 404 and display only part of the complete URL (whatever follows the slant mark beyond the domain name). When going to a new site, such as provided by a Google search, the initial page may show up but clicking on links within that page again brings up the 404 error. My only solution has been to uninstall whatever new version of Firefox has been automatically downloaded and then re-install version 3.6.0, which works.
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A possible cause is security software (firewall) that blocks or restricts Firefox without informing you about that, 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.
See Server Not Found - Troubleshoot connection problems and Configure firewalls so that Firefox can access the Internet and http://kb.mozillazine.org/Firewalls