The argument being presented would just as well support Microsoft releasing an automatic update to windows (automatically deployed to all their users) that only supported some tiny fraction of white listed applications. They then don't clearly make any promises about removing the white listing, or any timelines, but only vague statements about "ensuring a positive user experience" and "working with their application development partners".
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.