Actually, you can use it on Android, but it does require using Firefox Nightly[1].

That does come with downsides such as Nightly being less stable and installing extensions requiring some extra setup, so it isn't the best choice for everyone. But I myself am using Stylus (which works perfectly fine) and some other not officially supported WebExtensions on mobile this way and can recommend it if you don't mind the downsides.

[1]: https://blog.mozilla.org/addons/2020/09/29/expanded-extensio...