That indicates an extension is not working properly, possibly one from AVG.

1. Open the Add-ons manager (Click the Firefox button and then click Add-ons)
2. On the Extensions panel, if you have any AVG add-ons, click on them to select them and then click Disable
3. Restart Firefox to see if you no longer get the error

If you do not have AVG, you can repeat the above process with other extensions. For more details on troubleshooting this see https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Hi Tony,

Thanks for your help. I disabled two AVG add-ons and firefox is working again!

the problem isn't with avg, its with firefox. Am I supposed to disable a security program I paid for so I can use FF? FF worked fine until it went to version 4.

following the above instructions, i do not have any. so now what?

probasco,

You're wrong! The AVG extension is the problem, see AVG for an updated version of their Firefox extension.