Though it's not everyone's idea of sexy, InFormEnter has got to be one of my favorite Firefox add-ons. It's the ultimate time-saver for internet time wasters like me — anytime you reach a page with fields for you to enter, like shopping, giveaways, or registration, InFormEnter adds a little arrow next to each field. Click the arrow when you want to enter your name or other info, and your profile, with a list of common entires like first and last name, email address, phone number, etc., will pop up. Annoying fields get filled out, no typing necessary! The first time you use it, add your info into a field, and then click the "Add text to profile" in the pop up, so it'll be there for you every time afterward.
I used to bypass registration on pages for sweepstakes just because I had no patience to type in all my info, but it's a nonissue with InFormEnter now. And if you get annoyed with the little arrows appearing next to every field, you can turn it off and back on when you need it from its icon on your status bar!