I don't use KLP but a thought occured to me.

have you checked your privacy settings in Firefox. under tools >> options .... try unchecking "remember what I enter in forms and search bar" also try looking at password settings in the private data field, check to clear private data and open the settings button to make sure passwords are being cleared. you may also want to check the security tab and make sure the passwords field is unchecked.

I thought this might be the problem because I have firefox set to remember what I enter in forms. such as login user names to sites I frequent. at most sites I don't even have to type in a user name field. all I have to do is click on it and it will bring a list of user names I have entered. perhaps you have firefox set for the same and it is causing firefox to fill the form while KLP is also filling the form.