Since that's an extension, I imagine you have to deploy it with whatever method the browser uses for adding extensions. This I think should work with Safari and Firefox.
Firefox:
#!/bin/sh
User="$(who | awk '/console/ {print $1}' | head -n 1)"
#To install Firefox extension
sudo -u "$User" osascript -e 'tell app "Firefox" to activate'
sudo -u "$User" osascript <<EOF
tell application "Firefox"
open "/path/to/Firefox/extension.xpi"
activate
end tell
EOF
Safari:
Note in Safari you essentially have to create a simple HTML page where the page loads the Safari extension. Programmatically, I believe Apple disable other methods for deploying extensions as they want it to be deployed with end user interaction.
#!/bin/sh
User="$(who | awk '/console/ {print $1}' | head -n 1)"
sudo -u "$User" osascript -e 'tell app "Safari" to activate'
sudo -u "$User" osascript <<EOF
tell application "Safari"
open "/path/to/Safari/extension.html"
activate
end tell
EOF
And note that these methods I'm referring to are meant to be used in Self Service. I'm not sure how they will work if run via policies in the background. Also, I'm not sure if this method will work in 10.11. Lots of caveats. But hopefully it's a starting point for you to get something going. Good luck.
