Firefox runs on macOS
As soon as universal binaries/iOS apps becomes fully fledged on ARM Mac I can imagine Apple just removing support completely. It's almost inevitable the way Apple is progressing to basically move to the iOS style platform control to OS X.
Even from Mozilla's economics perspective, it doesn't make sense for Mozilla to be spending engineering effort maintaining both the iOS and OS X variants for something like 4% of desktop. If it was up to me, I would have just migrated/maintained the Firefox Wrapper iOS and called it a day.
I've said this before and I'll probably say it in increasingly irascible tones, but "next year macOS is gonna become just like iOS" is becoming the new "next year is the year of Linux on the desktop."
Yes, Apple will almost certainly keep making security decisions people (including me) don't like, but there's no compelling business decision for trying to lock the Mac down to only App Store installs. Yes, I know Apple gets 30% of software sales that way. No, that is not enough of a reason for them to take the hit to their hardware profits that would undoubtedly entail. Even if it were just a loss of a few percent of Mac sales -- which I think is extremely optimistic -- it'd be coming out of the sales of the most expensive Macs, and making up one lost Mac sale literally requires hundreds of sales on the App Store to make up for it. Any executive at Apple who suggested that would be beat senseless by their accounting department.
The real successful strategy is to do it very slowly and gradually. With each release make it a tiny bit more annoying to install and run non-store apps. Make them slower - "this app is not trusted so it needs to be monitored for your safety which might negatively affect its performance". Users will prefer store apps and will put pressure on app developers to publish on app store.
If they do it well then hardly anybody will even notice they finally killed the non-store apps.
Development houses now have an opportunity to bring their software to a larger market share (iPad, iOS and MacOS) whilst maintaining just one code base.
Development houses would also have much stronger piracy prevention (same kind of benefit that enables console games to function).
Development houses would have much stronger control over the running environment (preventing adblock, preventing VPN region bypass etc...).
Development houses would have much more control over updates.
Surprisingly just like the now defunct Mac Servers, Apple doesn't really care about the high end hardware Mac platforms. They only care for those graphic designers/video editor/MS office crowd and their software is coming in iOS forms.
