You need to keep browsing history for autofill to work. Make sure you have not enabled [[Private Browsing - Use Firefox without saving history#w_can-i-set-firefox-to-always-use-private-browsing|private browsing mode]] aka "Never Remember History" and aren't [[Delete browsing, search and download history on Firefox#w_how-do-i-make-firefox-clear-my-history-automatically|clearing history on close]]. You can boost a site's autofill score by bookmarking it and visiting it several times consecutively by typing in the address bar. Giving the bookmark a [[How to search IMDB, Wikipedia and more from the address bar|keyword]] will guarantee that it autofills even without history.