Other users have reported that this is caused by one of their add-ons. I suggest going to the Add-ons page and disabling everything that is not essential, and then restarting Firefox. Hopefully it's not caused by something you really rely on.

orange Firefox button or Tools menu > Add-ons

On the left side, click Extensions

Any luck?

Dear jscher2000,

Thank you for your prompt help. Following your direction, I disable 2 Add- ons related to Java but the trouble wouldn't go yet. I though the fast way to disable all add-ons is to uninstall the current Fire Fox 4. This time I install Fire fox 5.0 to renew every thing. And Voila, the trouble is gone.I use Fire fox all day today(Sunday June 19, 2011 without a hitch.) I appreciate your help.

NGMTR