The trick does work Rated 5 out of 5 stars

I just did it for tb 15. You have to modify three files (all the install.rdf), not only one, directly into the archive and then update the archive to save your changes.
I downloaded it from tb add-ons instead of fx add-ons. The xpi file contains everything for both tb and fx.

This review is for a previous version of the add-on (1.5.3).