In the M6 release of Mozilla, running under Linux (RedHat 6.0), the Tab key doesn't advance to the next field in a form. It does, however, defocus the keyboard from the input field it was in.
Seems OK on the 1999060408 build under NT; re-marking as [PP] to indicate that
it's platform-specific. Also changing area to Event Handling as (I think) this
is more to do with the keypress event than with a form submission.
Bug 7359 chronicles the same matter, and is marked to address All platforms:
*** This bug has been marked as a duplicate of 7359 ***
Yes, one of many tab/focus/form bugs. Marking verified.