This Partial Content code (206) may be sent from the server when the client has asked for a range (e.g. “give me the first 2MB of video data”).
It is vital for downloading data in chunks which avoids fetching unused resources. (I seldom watch a full video online.) Look at the outgoing request for a Range
header.
See Status Codes (RFC2616 – HTTP/1.1) (note that any 2xx code means “Success”):
10.2.7 206 Partial Content:
The server has fulfilled the partial GET request for the resource. The request MUST have included a Range header field (section 14.35) indicating the desired range, and MAY have included an If-Range header field (section 14.27) to make the request conditional.
And from the Range header section:
If the server supports the Range header and the specified range or ranges are appropriate for the entity
- The presence of a Range header in an unconditional GET modifies
what is returned if the GET is otherwise successful. In other
words, the response carries a status code of 206 (Partial
Content) instead of 200 (OK).
I do not understand the intricicies of caching and/or ETags or what to expect in FireFox, however.