After submitted this feature enhancement request to the Mozilla bug database, I received a comment on the bug informing me that the feature already (more or less) exists.
Here is my complaint: After using Firefox for several months, form fields with common names - email, address - tend to accumulate a very long list of auto-complete values. Most of them are not very useful and just slow me down. For example, see this street address field (from Senator Barbara Boxer's message submission form):
What I learned after submitting my enhancement request that it is possible to remove entries from this auto-complete list using a (in my opinion) completely undocumented feature:
Use the arrow keys or mouse to highlight an entry in the auto-complete menu, then press the 'Delete' key. That value will then be removed from the list of retained/suggested values! I would prefer to have an entry at the bottom of the list 'Remove all suggestions for this field' to completely clear the auto-complete list, but one at a time will work in a pinch.
This is particularly useful for those times when your credit card number seems to have been remembered by Firefox when it probably shouldn't!