The support.mozilla.org page that describes “auto form fill” contains the sentence:
After you’ve entered something into a form on a web page (such as a search box), the next time you visit that page, your previous entry will be available to re-use. (Emphasis mine.)
My testing, however, shows that you do not need to go back to that page for Firefox to present you with a drop-down menu showing previously-entered values. Any text-entry field on any page on any site will present you with a drop-down menu showing previously-entered values so long as the name of that form element is the same as the name of the element in which you previously entered those values.
Proof: go to https://developer.mozilla.org/en-US/play and into the HTML entry box, enter the text:
Now left-click twice on the text entry element that has appeared on the right, and FF will present you with a drop-down menu showing every value that you have entered into a “username” control/element, regardless of what site or page it was contained in.
Is this the way it is supposed to work (i.e. is it just the support.mozilla.org page that describes “auto form fill” that is wrong?), or is “auto form fill” really only supposed to present values entered into “this control on this page on this site”?