So here is our issue I'm having as well as several of our users.
All of us are using firefox 3.6.x - and having this issue since upgrading to 6.0.8.
When we create a new message - it pops up in a new window (which is how we have it set) - the cursor starts out in the TO: field, and as we start to type the autocompletes start to come in - and then the cursor will lose focus in the to field, and jump to another place.. so when we hit tab or try to select the user that is shown it ends up opening up the Add attachment file picker, or some other choice it happend to jump to.
Anyway to fix this easily without waiting around for a patch? :S