Secondly, Mozilla absolutely do have a responsibility to ship APIs that work to spec. It's not up to Addon authors to fix Mozilla's bugs (and that may not even be possible depending on the nature of the bug). And of course you're assuming Addon authors would be interested in going so far out of their way to support what is a relatively small user base.
It seems very funny to say "we had to give up and release a broken browser" and in the same breath take on extra responsibility for API correctness prior to letting any one who can't convince Mozilla to whitelist them to use the APIs. And take on responsibility of maintenance of a whitelist
We've been here before with desktop Firefox when the browser got a lot of flak for issues that were mainly caused by addons.
Their solution to that problem hasn't exactly led to a positive perception of the browser either, and it hasn't even left them the out of blaming it on anyone but themselves.
Besides, it would not be a good look to be blaming extension authors for Mozilla's problems. So I don't think that's a viable "out".
Most people would probably see this as a Very Bad Thing, for a few reasons. First of all, if we were to take the claim that it's "to ensure stability of APIs" at face value, it would indicate a staggering lack of maturity in their decision making process around releases. Next, we shouldn't take the "API" argument as being anything representative of reality: applications people develop (or browser extensions) have always been the responsibility of the developers developing them. If there were bugs in a browser, they'd discover and work around them, or they'd have a broken extension/application. There isn't a way around that. Perfect APIs don't fix it: people write bugs into their applications/extensions/platforms all the time. Things are broken all the time. Testing has never been Mozilla's responsibility. It's always been on extension authors. There are already ways to "declare" in an extension that support of a specific version exists.
There is no competent technical reason to end up in this situation. We're only left with incompetence and malice. And it's _very_ difficult to think that the people running a popular mobile browser could be so incompetent. Though I'm sure we've all said that before about other Mozilla decisions.