Firefox sometimes somehow refuses to save your settings. If you used more that one login info for a given site, firefox will stop auto filling your username and password. So, when you want to correct this, the procedure should be to go, from firefox menu, to: Tools > Options > Security > Show Passwords, then remove both login entries for that site. But sometimes, somehow, firefox fail to remove those entries. If you checked again, after restarting firefox, and found that both entries are still there, then you need to do a manual remove. Here is how:
Go to your firefox profile. Here is how to locate it:
in forefox address bar, type: about:cache
at the bottom of that page your cache directory is shown to you. And since your cache is located inside your profile, you now know the location of your firefox profile.
Now that you know where it is, open your profile, and open signons2.txt file, using a text editor.
different login data are separated by a line containing one dot. Locate the site you want to delete, and delete all the lines up until the next line with a dot. Save and exit.
Now restart firefox and go to that site, type your login info, and firefox will offer to remember what you typed, properly this time.