Is it possible to enable a Firefox addon using a terminal command on the mac?
What you need to do is create a new Firefox profile.
That profile will be completely different from the default profile that you normally use including the add-ons that are installed.
Command line Mac Command Line Options:
Symlink the Firefox binary (optional)
The Firefox binary is located in /Applications/Firefox.app/Conents/MacOS so I suggest making a symlink to it so you can launch Firefox from anywhere and you don’t have to type out the full path each time.
% sudo ln -s /Aplications/Firefox.app/Contents/MacOS/firefox /usr/bin/firefox

For the rest of the examples below, I will assume that the binary is symlinked.  If you chose not to, you’ll need to type out the full path.
Launch Firefox Profile Manager
The ProfileManager launches Firefox with a Profile selection box in the GUI allowing you to choose which profile you’d like to use
% firefox -ProfileManager


Create a Profile
This is the quick command to create a profile in the default location which will be in the ~Library/Application Support/Firefox/Profiles  directory.
% firefox CreateProfile &lt;profile name&gt;

To create a profile in a specific directory, you need to specify either the absolute or relative path
% firefox CreateProfile “profile_name profile_dir”


Launch Firefox specifying a profile
This command will launch Firefox with a specific profile
% firefox -P “&lt;profile_name&gt;“

Use this command if you have Firefox already running and want to have a new instance with a different profile
% firefox -new-instance -P “&lt;profile_name&gt;”