Primeiro, você não deve usar o Firefox - ou o X Windows - como root, pois é uma vulnerabilidade de segurança. Esses aplicativos devem ser executados com privilégios não administrativos, em cujo caso qualquer bug ou falha neles causaria danos limitados ao sistema.
Para responder à sua pergunta:
If you're putting together an installer that needs to be able to install extensions for a toolkit-based application, such as Firefox or anything using XULRunner, Gecko 1.9 (Firefox 3) introduces an easy way to do this for Windows, OS X and Linux. Windows users have also had the ability to install extensions using the registry for some time. Now, on OS X and Linux, you can simply copy the extension into a predefined directory on the user's computer.
To install extensions into these directories you must extract the extension to a directory with the same name as the the extension's ID. The ID is defined in the
install.rdf
of the extension, between the<em:id></em:id>
tags. For WebExtensions based add-ons the ID is the value of theapplications.gecko.id
property within themanifest.json
file.To install an extension to be used by all users on Linux, copy it to:
/usr/lib/<vendor>/extensions/<appid>/
Or...
/usr/lib64/<vendor>/extensions/<appid>/
Or...
/usr/share/<vendor>/extensions/<appid>/
To install an extension just for a specific user:
~/.<vendor>/extensions/<appid>/
(Fonte: link )