If librewolf is not a fork, then what is it?

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".

Does it matter? AFAIK Firefox doesn't plan to go closed-source, so applying patches should be continue being fine too.

It depends on how much Firefox enshittifies. If it's just about removing some telemetry configuration from upstream, then a couple of downstream patches will still do the job. If, say, Firefox decides to fully embrace the spyware business model and drop support for Manifest V2 in order to kill adblockers, then LibreWolf will probably have to maintain their own fat piece of logic built on top of Firefox. Keeping it as a soft fork would then be a lot of work (you'd basically have a patchset of tens of thousands LoC to keep porting through different versions of Firefox). And making it a hard fork would be even more work (it basically means that the LibreWolf folks are on their own and they have to maintain their own independent browser).

Maybe a hard fork would be more manageable if the scope was reduced to just web browsing instead of trying to be an app platform.

I feel that if Firefox up and died we would have people scrambling to work on something like this in much greater numbers.