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.
Not currently, but Mozilla has announced a “dramatic” increase in the available extensions for mobile coming in December, so, fingers crossed.
(as others have pointed out you can with Firefox Nightly and a custom collection)