Seems petty. Mozilla is one of the most pure open source projects out there in mission, ethos, and product. Addons are user choices, and trying to limit that choice is self defeating in free speech.
Well, some things at Mozilla seem a bit odd. A few weeks ago Nightly ended up having "Pocket" integrated, and rumour said it would replace Mozilla's own open source Sync plus the integrated Reader. Addons will soon no longer be able to be installed unless approved by Mozilla. (Another walled garden. Sigh...)
Bullshit. The add-on signing requirement will be able to be easily disabled. It's just as much of a "walled garden" as Debian's repos are (ie. nothing like a walled garden).