I just started getting this error too. It's only just started happening to me. I uninstalled Firefox 5 and reinstalled FF4, same issue.
I uninstalled everything firefox related I could find re-installed FF5 and still the same issue.
I haven't found a single solution to this on either the firefox forums/support pages or anywhere else. It's becoming annoying, as I despise using IE as it's incredibly slow and buggy on my laptop.
But, it's what I'm being forced to use currently.