Are you sure? Because I just created a barebones hello world. It works fine as a temporary add-on, but I can't install it from the file. Says the add-on is corrupted.

Then I downloaded a working add-on. It worked fine when installing from file. I removed the signing information. It installs fine as a temporary add-on, but throws the same error as the hello world.

I'm pretty sure that the add-ons need to be signed by Mozilla to install from file.