This is a plugin I found recently and really enjoy. It gives you the opportunity to redirect several webpages to alternatives. For example Youtube to FreeTube or Piped. If I search for youtube in Firefox, FreeTube automatically opens and shows me my feed. I like it, I use it, I can recommend it.
You can and have been able to install pretty much any add-on you want in the nightly and I guess beta versions, for a long time.
It’s not exactly convenient but it can be done.
https://www.androidpolice.com/install-add-on-extension-mozilla-firefox-android/