I\'m using a bookmarklet that inserts a script tag into the current web page.
This script has some UI and an \"input type=submit....
\" tag in it.
For Firefox you can find the default form styles by typing resource://gre/res/forms.css into the address bar, which does at least give you the details of the default style for an input button so that you can then copy them and override using a !important rule.
It's not entirely satisfactory though as the default style could of course change with a future version of the browser and the default style for different browsers might be different.