It seems like all addons are installed in %APPDATA%: when I start FF Portable for the first time, it creates a directory %APPDATA%\Mozilla\Firefox, and it saves all addons, passwords, ... in this directory.
I installed FF Portable, installed all addons etc on one computer, but when I try to run FF Portable on another computer, no addons are installed at all.
This is not really what I had in mind for a "portable" app. :S