This is very possible and you can do it in few very steps.

1-First you have to download the xpi add-on file instead of installing it.

2-Use the following command at the run prompt (type Windows + R): firefox -install-global-extension

For example if your downloaded xpi add-on is on C:\Users\Simon\Downloads\

Then you write firefox -install-global-extension C:\Users\Simon\Downloads

Note : The addon will be normally available where every addon is. Keep in mind that only administrators can uninstall it. Have a nice day Sincerely, Ahmed Nefzaoui

Thanks. But the addon is showing as disabled. I need it to be enabled by default. Also can I need the settings for the addon to be the same for all users.