Do you get a drop-down list when you start typing the first letters?

The website might be using autocomplete=off to prevent Firefox from filling form data like name and password automatically. Firefox won't auto-fill saved form data automatically when autocomplete=off is used.

There are some web pages that do not allow browsers to record or fill in the log-in or other information. But a Form Filling add-on still works.

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for what you want.