I have the same problem and I'm realy fed-up by now.
All the actions I did:
-In OS X 10.5 I deleted Firefox 16.0.2 completely several times after reinstallation.
-I did everything exactly what was advised here on this site, several times.
-I checked the whole computer with a virusscanner.
-I upgraded even to OS X 10.6
-installed Firefox 18.0.2
-The same problem remains:
User & Passwords not being saved