Could you try this: After clicking into the field with the unwanted autocomplete suggestions, press the down arrow key on the keyboard to highlight the suggestion and then press Shift+Delete (or sometimes just Delete) to remove it from Firefox's form history. Does that work? If that doesn't work, it's possible the site has asked Firefox to save a list in a cookie so the page can generate its own suggestion list. Here's how to clear that: While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites. Then try reloading the page. Does that help?