You need to install them in the firefox installation directory, which should be in /usr/lib/firefox.........The way to do that is to cancel after downloading it, otherwise accepting it will install it to your $HOME/mozilla/firefox directory............
You need to pay attention to the dialog box because sometimes the addon will only be installed in the main installation directory, or possibly in the user's home directory..........It should say something along the lines of "Press the accept button to install this extension in your user profile, else press cancel to install it in the firefox directory"........Maybe not those exact words, but you get the idea.....

HTH
---thegeekster