You may be using a beta version. In 1.0 there's a drop-down above the addresses in the Compose dialog to switch between accounts for sending the mail.
Well that's certainly good news. I'm glad they changed THAT.
Plus, it forces me to send all email using the default email account SMTP location. Yes, it does allow me to "send" the mail through the account I want to, but the mail is actually using the SMTP for a different account.
Unfortunately, I'll have to stay with Outlook until the Mozilla dudes and dudettes sort all this out.