If I've entered "dbaron@moz, mats.palmgren@br" into a form field, and then I start typing "bzbarsky@mi, mats" into that same form field, it would be great if Firefox would offer to autocomplete mats's address. This would be useful for web-based email (to, cc, bcc) and Bugzilla (cc, keywords). Gmail notably implements something similar for its own autocomplete feature using address book data, but most sites aren't so AJAXy.

I think we would only do this with @multiple as we would be sure it's expected: bug 607948