sayrer noticed that the Gmail search field was triggering form history DB searches, but no results ever show up. The immediate reason for this is that no results are in form history (the fieldName is ":ra"), but the real question is why these values are not being saved. If I manually store a result in formhistory.sqlite, I do get an autocomplete dropdown containing that entry. There's no autocomplete=off on the field, and I do see nsStorageFormHistory::Notify being called. It iterates through a number of form elements in the submitted form, but doesn't seem to attempt saving any of them. Over to MattN to investigate what's happening here.
After some investigation, I have found that the search field (with name ":ra") is not being submitted directly. From what I can tell, a form with hidden input fields is being constructed with JS dynamically and that is what is being submitted. The search term is actually being sent in a hidden <input> with a different name ("q") than the search field (":ra"). Since we only save input fields of type "text" and the field name that is getting submitted does not even match the search field I'm marking this bug as invalid.