For several years, FastMail has allowed you to use Ctrl-Enter on the Compose screen as a shortcut to send a message. Unfortunately Firefox
1.5 and 2.0 both have a bug that means this doesn't work as expected, instead showing the "Download manager" dialog as well as attempting to send the message. This bug in Firefox is documented and being tracked at the bugzilla site here.
1.5 and 2.0 users, this is a good work around until this bug is fixed, just use Ctrl-Shift-Enter instead.