I think the password manager in Firefox should add support for changing a password. Ideally I would be able to right click on a password box and have the option to generate a new strong password, on a site where I already have a stored password (that is now expired).
I use a site that forces me to change my password about once a month. Currently I have to use a 3rd party tool to generate a new password, then copy paste it into both the site and Firefox password manager.