Where are you installing from, and how are you trying to install it?
I downloaded passwdmaker-1.4.3.xpi from
passwordmaker.org
Downloads
Firefox/Mozilla/Netscape
I dragged passwdmaker-1.4.3.xpi into Firefox, just like I did will all the other extensions I installed.
Just clicking on the link "Download and Install PasswordMaker 1.4.3" results in a page showing the message "You don't have access to this content (HTTP Error 403).".
I am running the Dutch language version of Firefox 1.5, on Windows XP Pro SP2 (also Dutch). I tried this both at home and at work (both systems have the same versions of Firefox and Windows). I tried re-downloading passwordmaker.