We nerds have our favourite editors, mine is vim. Each one of us is most productive while using the usual editor. It would be fine to be able to select in preferences a program to run into the mail composition window. Also, commercial implementations might build on this feature to further customize their versions with specialized editors.
Yeah, that would certainly be tremendously useful. In particular because editors like vim have very good ways for reformatting email messages, while still keeping the quoting intact. Mahogany and (hell!) even M$ Outlook can be talked into having an external editor. My MUA of choice (mutt) of course too. As for implementation strategy, Mahogany allows you to press Ctrl+E in the normal editor Window, where it will then fire up your configured external editor (with a temp file). Once you close the external editor, the temp file will be loaded into the integrated editor. That shouldn't really be difficult to implement for Mozilla.
*** Bug 175475 has been marked as a duplicate of this bug. ***