There are good technical reasons for requiring signed add-ons. Well, maybe not so much "good" but necessary because of other bad things in Firefox that prevent a less extreme requirement from being implemented.
But the signing requirement isn't what upsets anyone. It's that add-ons must be signed _only by Mozilla_. The whole mess could have been avoided from the start by saying that add-ons must be signed by a trusted certificate but the end-user gets to choose what certificates are trusted.
There are good technical reasons for requiring signed add-ons. Well, maybe not so much "good" but necessary because of other bad things in Firefox that prevent a less extreme requirement from being implemented.
But the signing requirement isn't what upsets anyone. It's that add-ons must be signed _only by Mozilla_. The whole mess could have been avoided from the start by saying that add-ons must be signed by a trusted certificate but the end-user gets to choose what certificates are trusted.