Don't know. I just spotted that it's the only meaningful difference I had in my traffic between visiting with Firefox and with Chromium. IIRC, it's the fetching of the capture timestamps that fails when the Referer header is missing.
> I haven't changed any settings from the Firefox default
You might have forgotten like I did once.
> and allowing it breaks links to jwz's site.
Do you mean this one?
Seems to be working fine for me with the Referer header enabled. Maybe it was a temporary glitch?
> The HTTP referer (a misspelling of referrer) is an optional HTTP header field that identifies the address of the webpage (i.e., the URI or IRI), which is linked to the resource being requested. By checking the referrer, the new webpage can see where the request originated.
https://tools.ietf.org/html/rfc7231#page-45
> The Referer header field allows servers to generate back-links to other resources for simple analytics, logging, optimized caching, etc. It also allows obsolete or mistyped links to be found for maintenance. Some servers use the Referer header field as a means of denying links from other sites (so-called "deep linking") or restricting cross-site request forgery (CSRF), but not all requests contain it.