When you say "emailing via Firefox", what do you mean? Are you accessing your mail provider's website with it, or are you using a firefox e-mail plugin?
If it's the former, and it's a form on a web page you're trying to type in, or maybe some java applet, this is actually quite common, as a lot of the more popular on-line companies are still using software and scripts that aren't fully Mozilla compatible - it's shocking really. Probably some back-hand bung from Microsoft going on.