HOWTO: Save a Lily patch as a Firefox add-on

From LilyWiki

Open the patch you want to save and select "Save as Add-on" from the File menu. A project metadata dialog will appear. Fill out all fields. When you're done filling out the dialog, click Save. A file save dialog will appear. There should be a suggested filename already supplied. Choose a location to save in and click save. When the build is complete, your extension should appear in the location where you saved it. The .xpi file can be installed as you would normally .