If you don't mind typing.....
I really don't think that auto-complete passwords are even close to an idea worth pursuing.
You can pretty much dump most to all the cookies. (The only cookies I have to keep are for my Verizon, Yahoo services).
I prefer to shop at say Amazon without the b******t greetings and recommendations et al.
Give me anonimity until I actually want to buy something, then trust me after you sign in, their server remembers your credit card number and shipping address like you've been best friends for ages.
You try to be nice, and let sites set cookies, and they put them in with expiration dates extending to the middle of the century. Oy, check keep cookies until I leave Firefox. The next trip to the website you'll be able to make friends all over again.