Try this extension. https://addons.mozilla.org/en-US/firefox/addon/saved-password-editor/

I went to the account web-site for which I want to store the username (a number) and the password (alpha-numeric). With the cursor in a data box, I right-clicked. I chose to save login info. A dialog box appeared, and I filled in the username, password and submitted. The info was added to password manager's list. I closed the list. I closed the browser. I re-opened the browser and went the the account's web-site. The login box appeared as usual. I entered the first character of my username. As had been the case before I downloaded the add-on, the rest of the username filled in automatically. I tapped the down-arrow and then the tab key. The password's asterisks filled in. This was the major improvement. I still had to fill in the first character of the username, then tap the down arrow, then tap the tab key, and finally the enter key. It is helpful to have the password filled in, but I expected that both the account number and the password would be filled in for me when I booted the web-site from my bookmarks. How can I make this happen?

Try doing a click in the username field, then click on the correct username from the suggestion offered; then the username and password should auto-complete.

Thanks! I expressed my appreciation in a donation, not huge but well intended!