This is only possible on Firefox Nightly, and you have to install them by making a Mozilla account and creating extension collections[0].

This is frustrating to do, when the desktop site didn't allow me to add a mobile-only extension[1] and I had to figure out how to do so by modifying HTTP requests. This is a reversion from older Firefox versions, where you could install any extension via the web store as you would on desktop.

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

[1] https://addons.mozilla.org/en-US/firefox/addon/google-search...