Thanks to Asa Dotzler who has left a comment explaining the signing process for Firefox addons in more detail. Here it is in case you haven't seen it on the original post.

"The signing process doesn't have anything to do with ensuring addons haven't been changed. Signing is to let you know who is providing the add-on, but if you get the addon from Mozilla's add-on site, we verify all of that for you so all you have to do is trust Mozilla, not necessarily every add-on author."

I'm glad to clear that up.