A possible cause is security software (firewall) 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 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.

See:

• Server Not Found - Troubleshoot connection problems
• Configure firewalls so that Firefox can access the Internet

Take a close look at any add ons that may be conflicting with the tabs displaying the names of the pages. I had the same problem and disabled some and the problem went away. I can't tell which one solved the problem because I have to run. My cat is on fire again. It is good to be Dangerman.

I had the same problem and tried disabling all the ColorfulTabs add on. That didn't help. The problem was seemingly solved when I removed ColorfulTabs and went to their website and downloaded the latest version. The website is http://www.binaryturf.com/free-softwa.../colorfultabs-for-firefox/ . I installed the version for FF5. Everything is working fine now. Good luck. This is a very frustrating problem. It really messed with me in an organizational sense.