I had a similar problem and found that it was caused by McAfee SiteAdvisor. If you have that extension, disable it and see if the problem goes away. If you don't have Site Advisor, disable all your extensions and see if the problem goes away. If the problem is gone, then add the extensions back one at a time until the problem returns.

Good day to all. Likewise I am also having the same problem with Firefox and McAfee SiteAdvisor. No matter what I use for my intial "home page", whenever I do a search ... Google or Yahoo, etc., or navigate to a "favorites" URL, I cannot return to the "home page". The "BACK ARROW" is grayed out. If I should navigate a second time for the first navigation, then the back arrow is then available. In for Firefox to work properly, I have to disable SiteAdvisor. I though just maybe with Firefox v4.0 the problem would somehow be corrected. But sadly .... NO. I don't know if the problem is with Firefox or SiteAdvisor. This problem started after version 3.16.14 was released. Prior to that SiteAdvisor and McAfee were getting along fine! I sure hope that someone in the developer's department reading this !