Is it radically different? It’s a feature that iPad OS supports that the iPhone version of iOS doesn’t, and I don’t think Android does (though I’ve not used an Android tablet in nearly 10 years, so maybe tablets on Android can do it?). Obviously desktops all support multiple windows and have done forever. Technically, by not having implemented this feature it actually means it’s more similar to Android.
Firefox is rather under-resourced in terms of developer power, and they’ve been consistently prioritising other things rather than implementing this feature. I don’t think there’s much more to it than that. It’s a perfectly reasonable explanation for why they haven’t done it—any team needs to prioritise what they work on. But it’s also reasonable for a user who values that feature to choose a competitor that has delivered it over one that has not. That’s the natural trade-off.