Since it isn't signed/reviewed, you need to load it in FF using Temporary addon page.
Addon will be only be installed for the browser session.
You'll need to reload it again if you close FF and reopen it.
If that is not your issue, you may need to include the following in your manifest.
"id": "YOUR EXTENSION ID HERE"
Create an ID if you don't have one.
Something like "firstname.lastname@example.org" should work.