Sxipper is the autofill manager to rule them all, allowing you to create various profiles which store names, usernames, passwords, and every other piece of information you end up entering into sites all the time. The method Sxipper uses for this is rather ingenious, as the add-on is trainable and the network of users are its trainers, allowing new forms, login screens, and other annoying fields to be added as the extension is used all over the world. Trainers are ranked, and thus people who train forms poorly will lose the ability to keep training.
While it can be a bit intrusive at times, that is mostly when it is trying to learn a new form or is confused over non-standard inputs. When you install the plug-in, it imports your saved data from Firefox and populates a profile for you, which you can then fill in manually to add anything else you want to keep stored. From then on, every time there is a form or login screen, Sxipper will pop up and give you various options. This makes filling out profiles for new accounts on websites simple and easy as you simply click once, make sure everything is filled out correctly, and hit submit.
Sxipper can learn any type of field, but sometimes it does get confused. Sites which use frames and load multiple pages with forms will make the add-on freak out, trying to put up multiple profiles at once. Thankfully, you can simply click the little "x" to dismiss it and it calms down.