Hi tony.nield, This article should help: Password Manager - Remember, delete, change and import saved passwords in Firefox Scroll down to Viewing and deleting passwords.

Note that you can double-click the username and password field in the Password Manager to edit these fields and remove the password part,

Thanks for the information on password manager which I am thinking about using, but I did not find anything which will remove established prompts. Perhaps the only way to do this is to reload Firefox