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 either install it unsigned with Firefox Developer Edition and it will be permanent. Or you can sign it yourself (you don’t need to publish it on AMO): https://extensionworkshop.com/documentation/publish/signing-and-distribution-overview/ and it will work on regular Firefox.