https://searchfox.org/mozilla-central/rev/1c54648c082efdeb08cf6a5e3a8187e83f7549b9/dom/media/ChannelMediaResource.cpp#203,226,241,244

Since bug 1418430 was fixed, Gecko has intentionally accepted 206 Partial Content responses that do not fulfil the complete range requested. Such responses have a Content-Length header indicating the number of octets in the response body, which should not be interpreted as the entire resource length.

The effect is a media resource truncated at the end of the first partial response.

The check for zero mOffset would have been necessary when using Content-Length from (fully request-fulfilling) Partial Content responses, but is not necessary for 200 OK responses nor when using the complete-length field from Content-Range headers.

https://hg.mozilla.org/mozilla-central/rev/f2306039790583a2c1f75888010d044600d75f82 moved some Content-Length usage to Content-Range.

and use resource lengths available even when not the initial request.
This has the effect of using the most recent indication of resource length.