That's definitely not how Apple works. They'll just break it if they want to, though the change would probably show up in a beta OS first.

Apple would only break this if there was significant benefit to them for doing so.

I'd actually bet nobody at apple cares if they break Firefox users, they don't have the backwards compatibility culture of some companies like microsoft, especially for undocumented apis.

Don’t, you’ll lose your money. Firefox is popular enough to be on their list of apps they try not to break. And the culture you’re talking about does exist, they’re just less insane (and chatty) about it.

If you use an undocumented API and it breaks, Apple will point at you and laugh. Then they will use the example to tout the App Store.

Especially if you're an org with the pedigree of Mozilla.

Except that if they don't realize FF is using it and decide to drop it completely in their next update. That'd be a fun day for FF users.