How to install firefox extensions for all users 


Unzip xpi to /usr/lib/firefox-addons/extentions.

Make sure permissions are 755 to 775 for dirs and 644 to 664 for 
files (recommend just setting umask to 0022 while installing).

The trick is to name the directory as the extention ID is called, 
which is usually something like {2JBUJUB2J4BJ262B42J2} "jibberish"

This can be found in the install.js file if there is one.