Rated 2 out of 5 stars

in each case i tested, it doesn't work - it seems that if FF doesn't prompt to remember passwords, then this extension can't fill them in either, even after entering the credentials manually (the auto-guess from the current page fails to find the input fields, even though they are easily found in the page source)

This review is for a previous version of the add-on ( 

I'm sorry to hear it can't help you. Please understand that many log-in forms break Firefox's Password Manager (or perhaps you could say it's already broken to some extent). This is documented in the FAQ on my support site. Unfortunately, you will have to use some other tool. I usually recommend iMacros for Firefox: https://addons.mozilla.org/addon/imacros-for-firefox/ . I also recommend using its Master Password feature to protect your passwords stored in macros.

Note that SPE is not, nor will it ever be, a password manager, and as such, changing Firefox's password filling behavior is outside its scope.