Instant Submit allows you to specify a target form on an html page to submit as soon as the page loads, and your password manager has had the chance to auto-fill your username and password!

All you have to do is put "instantsubmit=x" in the query string (creating one, if necessary) of the url, where "x" is the form you want submitted, starting from 0 (0 is the first, 1 is the second, etc).

instantsubmit.user.js version 0.5

Greasemonkey