I thought there was a discreet way to edit and/or add passwords in Firefox but having looked all over the knowledge base it appears that isn't the case. Maybe I was thinking of some of the older versions of Firefox.
I know that you can check the pref is set correctly for Firefox to remember passwords by typing
about:config in your addy bar, then type
signon in the filter bar and then you'll see the pref -
signon.rememberSignons. This should be set to
true. If it is set to
false you can reset it by double clicking or right click and
modify.
In
this link to the Knowledge Base you'll see entries for details regarding Passwords.
So, I'm quite surprised you can't edit your passwords and sorry I couldn't be more help.