How do I make Firefox save form data for websites that specifically disallow it?

I was able to get around this restriction using the LastPass password manager and it's associated Firefox Plugin. It's able to save log-in information from websites that FireFox alone would not. It'll also save other form-fill items like address, phone number, etc, if you wish, as well as generate and save very secure passwords for you, semi-automatically. I highly recommend it.

Try Form History Control, it can store form fields on any site.

I use it frequently and it helps a lot with remembering things, but you need to configure it in a way to work it with everything:

(Click here for a full-size version.)

The Firefox Plugin Web-Developer Toolbar provides an option to (re)enable autocompletition for disabled form fields. Once installed, being on the website that obviously has autocompletition disabled, on the Web Developer Toolbar go to Menu Forms, enable autocompletition. Then fill in the form, and it will ask you to save it like a normal form.