Install add-on as xpi file instead of unpacking to folder

[her34]

Firefox 4 has allowed keeping addon as xpi file which helps firefox start faster.

https://developer.mozilla.org/en/Extensions/Updating_extensions_for_Firefox_4#XPI_unpacking

[arantius]

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.