Click the 'hamburger' at the top right of Firefox. (Three horizontal lines).
Click 'Add-ons'.
Search for 'form filler'.
Look for one that, at first glance, might seem to meet your needs.
Click on 'More' to read all about it (and, probably even more important, to see the review rating).
Click on the number of reviews to read detailed comments from users.
Repeat until you find the best one for you and then click on 'Install'.