When viewing your Saved Passwords put the domain name in the Search field at the top to see only passwords for that domain. Then click on the Username field header, so it is highlighted, to do an alpha/numeric sort of the Username field; same works with the Passwords field. Clicking on the highlighted field header again and it will toggle between "down" sort and "up" sort.

Unfortunately, that is only a temporary sort. It will disappear when that panel is closed. You could look for an add-on with a feature like that, but I have never seen one. The Saved Passwords window defaults to displaying by the domain with an esoteric alpha/numeric sort order separated by the colon and the slashes; sort order is numeric followed by alpha and you need to study it hard to see the pattern of how it does the "sorting".

THANK YOU...After I upgraded to the latest version of Firefox all my saved passwords were in alphabetical order.

Do you want to alphabetize the names in the drop-down list of the user name field on the web page?

That indeed seems to have changed in the current release and items in the drop-down list are now alphabetized by default.