I think their point is that they build on top of Firefox. If Firefox died tomorrow, Librewolf would die with it.

It’s a downstream patch.

Changes are recorded, then applied to the latest upstream version (Firefox) then packaged and sent as an update (LibreWolf)

In other words, a fork?

Or how does the semantics of patch/changes/update differ in any way from maintaining a fork? The packaging has nothing to do with this. It may not be a well-maintained fork, the maintainers might not see themselves capable of adding features and bugfixes on their own, but a fork is a fork.

Wikipedia calls it a fork

https://en.m.wikipedia.org/wiki/LibreWolf

It may just be semantics though.

I think the distinction is a soft fork or a hard fork. A soft fork is just a patchset over upstream and some people do not call it a fork. Hard fork mean that you are diverging from upstream, you do not longer follow upstream and you are fully separate from the upstream

I think fork implies following upstream. If you don't follow, then it's a divorce, not a fork.

Your definition of fork excludes many notable forks. I never heard someone call a software project a divorce.

No, "fork" means development has separated paths from original project. Changing some relatively minor parts but otherwise keeping close to upstream would be better called a "spin", especially when most of the changes are just to the default configuration.

It's a shame that GitHub messed up this term by calling any clone of a repository a "fork".