Firefox 6 is a brand new version, only out now for a few weeks. It's possible that as with all new browsers there are bugs that will be fixed along the way.
I'm not able to look for instructions on clearing the cache on Mozillas's site right now for you but maybe someone will look it up to help you out. Usually however, in the 'help' and then 'about' you'll find the version and there will also be help topics within the help menu for things such as clearing your browser.
If you are unable to resolve your browser issues then maybe roll back to a previous version or try a different browser.