i think it's because it will not pass AppStore review

If you want to have a browser with a different engine you need a whole different developer account org, you need to maintain two different code bases and you need to very specifically set everything up to pass all of Apple's tests, which are incredibly onerous in time and work, so it doesn't make sense for Mozilla to do so for such a small market share (hell Google doesn't either, and they're a lot bigger than them)

Long read, but it exactly explains why: https://open-web-advocacy.org/apple-dma-review/

One reason is that Apple does not permit regional binaries, so you cannot use the same listing in the EU/Japan/Brazil/whatever other place may force Apple to act sociable and the rest of the world.

Creating a separate app would work, but all existing Firefox users would have to download a second Firefox browser app, probably sync their accounts if they want to keep their data, and then remove the old one manually. You'd end with a Google Meet/Microsoft Teams situation (where one app is labeled "new" and it confuses the hell out of everyone).

Furthermore, developers cannot actually use the released app they've made if they're in the US, where a lot of Firefox devs are.

Then there's the (what I can only presume to be illegal) Apple Tax you need to pay to distribute an app outside of the app store (which is what the Github repo linked is doing), which is an amount paid per user that downloads an app outside of the app store. Epic has promised to cover that cost (out of spite, probably) for one of the major alternative stores, but if they go back on their promises you're suddenly paying Apple so people can use your free app on the phones they bought.

There are also other issues (Apple's arbitrary testing requirements, for one); Apple has once again succeeded in implementing the law in such a way that it's impossible to exercise your rights. Until the next big Apple lawsuit about this, I don't expect browser companies to bother with a non-Safari overlay.