https://tools.ietf.org/html/rfc7234#section-4.2.2

   Since origin servers do not always provide explicit expiration times,
   a cache MAY assign a heuristic expiration time when an explicit time
   is not specified, employing algorithms that use other header field
   values (such as the Last-Modified time) to estimate a plausible
   expiration time.  This specification does not provide specific
   algorithms, but does impose worst-case constraints on their results.

The standard does not say what heuristics should be used, so Firefox’s heuristics is no more “legitimate” than Chorme/Safari’s heuristics of expiring immediately when Content-Dispostion is present.

“Firefox legitimately treats...” is highly misleading, making it sound like Firefox is more standard-compliant, but it’s not; it’s just different.