Receiving this error "TypeError: Components.classes[cid] is undefined" when opening Firefox 4. Never received this prior to upgrading to version 4.
Also receiving this error when logging on to some websites. Able to close the error screen and continue but it is aggravating.
That indicates that you are using an add-on that is not compatible with Firefox 4. For details of how to troubleshoot this to find out what one is causing the problem see https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes