Is "auto form fill" (browser.formfill) supposed to be "cross-site"?

Firefox Development
#1

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:

<input name="username"/>

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”?

#2

It has always worked cross-site, and I strongly believe that is intentional.

I don’t believe the support page is wrong. The page says that it will fill in the fields on that page the next time, but doesn’t say it won’t provide the same prompt on other pages or sites. It seems to me it wouldn’t hurt to add the information, but I believe the writer tried to keep the page simple to understand and not add additional information that might confuse someone.