This may be a problem with an add-on.

Try Diagnose Firefox issues using Troubleshoot Mode

If the problem disappears it's an add-on - try disabling them to see which it is.

Safe mode does not fix the issue, nor does uninstalling firefox, removing all traces from the system and reinstalling.

Version installed from CentOS repositories:3.6.4-8.el5.centos

I don't know what this is. I would suggest a lot of googling and asking on Centos forums.

Have you tried setting up a new profile - see Troubleshoot and diagnose Firefox problems

Does it happen whenever you start up? Was this an upgrade from a previous version? If so I wonder if there's a dependency problem in the Centos rpm - perhaps a mismatch with xulrunner. You could try reinstalling xulrunner (or updating it if there's a later version). I had a similar problem with SUSE rpms recently.

Or you could try installing the Mozilla tgz for comparison. If you do that set up a new profile and keep the two versions separate (or uninstall the Centos one)

That's all I can think of for now.

I recently had this problem and noticed that the "Work Offline" option had somehow been checked (in the drop-down menu when Firefox is clicked in the uppermost left corner of the page)...I unchecked that and the errors disappeared.