If you're having problems with password autofill on Firefox 22.214.171.124, read this forum post.
signons.txt has been replaced by signons2.txt, which starts with a '#2d'
Sections are separated by a "."
A section starts with a url, followed by 2 lines each for a username and password, and then one line for domain. If this line is empty, and you have stored more than one username, password autofill would not work.