How do I install one extension for all Firefox profiles?
You need to put the global extensions (.xpi files) into the /Applications/Firefox.app/MacOS/extensions folder (when you select Firefox.app, right-click and "Show package contents").

If Firefox doesn't recognize the xpi, try to install it for your profile, then move it from ~/Library/Application Support/Firefox/Profiles/&lt;profile&gt;.default/extensions to the path mentioned above.