There appears to be a bug in firefox 1.5 that causes the download manager to appear when you use the ctrl-enter key combination on the compose screen to send an email. Unfortunately I can't see an easy work around for this at the moment, we can just hope that it's fixed at some stage.

I've just created a bugzilla report so the firefox developers know about it. If you have a login at the bugzilla system, please vote on the bug to try and increase the chance of it being fixed.