What is a fork? Is it an extension or different app altogether?
It’s a different app. Imagine the prongs on an actual fork, they all stem from the same root. Forking something basically means taking that root and continuing to develop it. You’ll keep working on your version, and the original developer might keep working on theirs. Sometimes you’ll end up with similar looking prongs, and other times they’ll vary wildly between themselves. What I assume the OP means by “librewolf is a Firefox fork” is that it’s most of the code base for Firefox, with most of the same features, but also with some added privacy settings that are enabled by default.
Disclaimer: I don’t know anything about librewolf itself. That being said:
https://librewolf.net/installation/ - you should find a version for your OS here.
It’s a different app. Imagine the prongs on an actual fork, they all stem from the same root. Forking something basically means taking that root and continuing to develop it. You’ll keep working on your version, and the original developer might keep working on theirs. Sometimes you’ll end up with similar looking prongs, and other times they’ll vary wildly between themselves. What I assume the OP means by “librewolf is a Firefox fork” is that it’s most of the code base for Firefox, with most of the same features, but also with some added privacy settings that are enabled by default.