No idea. With flash this certainly used to work (i.e. Adobe distributed an APK with the Flash plugin, and other browsers including Firefox were able to use the library contained therein), but of course Flash has been discontinued for quite a while now and I don't know whether a similar approach might still work with the most recent versions of Android.
In any case though this would raise the backwards compatibility constraints on Mozilla quite a bit, and I'm not sure whether they'd be willing to take on that burden. Of course even now they can't go around breaking GeckoView-APIs all willy-nilly, but when it does happen, it "only" affects app developers the next time they want to recompile their app with a newer GeckoView version.
If GeckoView was being distributed as an independently, user-installable library instead, any breakage at all becomes a big no-go, because users naturally expect their existing GeckoView-using apps to keep working regardless of which updated version of GeckoView they might happen to install.
In any case though this would raise the backwards compatibility constraints on Mozilla quite a bit, and I'm not sure whether they'd be willing to take on that burden. Of course even now they can't go around breaking GeckoView-APIs all willy-nilly, but when it does happen, it "only" affects app developers the next time they want to recompile their app with a newer GeckoView version.
If GeckoView was being distributed as an independently, user-installable library instead, any breakage at all becomes a big no-go, because users naturally expect their existing GeckoView-using apps to keep working regardless of which updated version of GeckoView they might happen to install.