You can clone with
HTTPS or Subversion.
Firefox 4 has allowed keeping addon as xpi file which helps firefox start faster.
Essentially, dupe of #1187.
We still support Firefox 3. This will be practical some time after we don't. Until then, we still want the inner jar, to provide this same sort of benefit to Firefox 3 users.
Do not <em:unpack> nor JAR-ify.