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.
I went through the issues. Indeed what I said was all you need for redirecting from YouTube et al, but now it also checks every single libre instance you go to and goes to somewhere else if the instance is down.