Also when you start on an old version of Android it's much easier to incrementally add support for the newer features with API version checks. But if you start a brand new app with a target of Android 14, it's gonna be feel like you're just throwing compatibility hacks all over the place to support older versions. Similar end result, but the perspective of the second one makes it look much dirtier than it really is.

Firefox was around when Android 5 came out, so it makes sense that they'd keep up with new features without rewriting the old stuff, so no need for them to raise the minimum SDK, only the target SDK.

A lot of apps also rely on frameworks like React Native, Cordova, Ionic, Xamarin and whatnot that also only target some ranges, so a good chunk of apps could run fine on older versions if the framework hadn't dropped support for it.

But from a code perspective, you could build for minSdk=1 and targetSdk=34 and run well on both without sacrificing anything for the modern versions.