this can have various reasons:

first of all make sure, that your firewall/security software is allowing firefox to fully connect to the internet.

then you might try out the solutions (proxy settings, change of dns server) provided in this help thread: https://support.mozilla.org/de/questi.../704934

if it does not help, also try disabling ipv6 like it is described here: https://support.mozilla.org/en-US/kb/Server%20not%20found?s=server%20not%...

I HAVE THAT PROBLEM ALSO. IT USUALLY TAKES ME 5 MINUTES, AFTER CLICKING "TRY AGAIN" TO ACCESS MY HOME PAGE, WWW.AOL.COM. I AM FED UP WITH THIS PROBLEM AND UNDERSTAND THAT OVER 3000 USERS HAVE THIS PROBLEM ALSO. I WOULD LIKE TO INSTALL THE NEW VERSION OF FIREFOX, BUT WHEN I GO TO ORGANIZE OR SAVE MY BOOKMARKS, IT SHOWS MY PASSWORD FOR EACH SITE, NEXT TO THE ADDRESS. I'M JUST FED UP!!!!!

Tools -- Options -- Advanced (tab) -- Network (tab) -- Settings (button) -- "Auto-detect proxy settings for this network" -- OK...... then restatre Firefox

I have done absolutely everything that has been recommended - changing proxy settings back and forth, changing ipv6 settings back and forth, disabling addons - yes, back and forth. Firewall is fine.

This problem started when FF upgraded to the latest version.

"Firewall is fine. "

Not saying that your problem is necessarily with your firewall, but why not read this posting over at MozillaZine from back in 2006.

The top 12 ways to get fooled by Firewalls:
http://forums.mozillazine.org/viewtopic.php?p=2295421#p2295421