How do I install one extension for all Firefox profiles?

I run many firefox profiles simultaneously. Since moving to the mac, I've lost the British English dictionary. I can install it as an extension on each profile, but I'd like to install it once across all profiles.

I've tried firefox -install-global-extension "/path/to/extension.xpi" but that appears to have no effect whatsoever. I've tried copying the installed extension into various folders in /Library and ~/Library but nothing has worked.

How do I install a Firefox extension once so it is active for all profiles on the computer (or this user account, either is good)?

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/<profile>.default/extensions to the path mentioned above.