I've also tried uninstalling and reinstalling and that has not helped.
There is a support article directly on point: Websites say that Firefox is outdated or incompatible even though it's the latest version. Does that work?
You have a corrupted user agent (Fast Brows Search: FBSMTWB) that identifies you as Firefox/3.5.5
You need to reset the general.useragent.override pref on the about:config page.