From pentest list:

I would like to announce the release of Groundspeed, an open source
Firefox add-on to support manual webapp input validation testing.
Groundspeed allows you to modify the target web application's
interface by manipulating the forms and form elements loaded in the
page, in order to remove client-side controls and limitations.

Some of the practical uses include changing hidden fields, select drop
down lists and other fields into text fields, removing size and length
limitations on input fields and modifying JavaScript event handlers to
bypass client side validation without actually removing it.

You can find more information and download Groundspeed here: