I use the one that comes as part of Google Toolbar. You can have multiple profiles, which is handy, though it sometimes get mixed up with things like UK post codes and US zip codes. On the whole, it's pretty useful
it does it itself up to a point, for instance, if i type "S" into a username form it brings up "severedsolo" which is my username for nearly everywhere
My Computer
Computer type
PC/Desktop
Computer Manufacturer/Model Number
Medion Erazer (note to self: insert model number) - with custom additions
i suggest that you not use one of the addons, and you disable all autofill features in firefox. in my book the security is more important than the time i would save.