To the best of my knowledge the only way to do this is to enter your details and make a note of them and select `Not this time`; exit the site and re-enter by typing your details carefully again and selecting `Remember`. That way you can validate your details the first time and save them the second.
If you do want to edit details you can do this in `Documents and Settings/YOU/Application Data/Mozilla/Firefox/Profiles/****.default/signons2`. However it is encrypted so the editing will necessarily take the form of deleting the details for the specific site. The next time you enter the site you will be asked if you want to save your details.