Please go Start > All programs > Mozilla Firefox > Firefox (Safe Mode) and browse to the email service you're using. Most likely there is something wrong with the extensions of your browser which in this case you should disable everything that you don't know. Usually these errors are related to Javascript and since almost email services use Javascript for their email interface this could be the problem. Manually use the Add-ons/Extensions menu and disable one by one until the browser works as it should.
