It's in the spec that the browser is allowed to pick a duration if you don't tell it one, so you know that you don't know how long it is cached if you do not set a header.
Both browsers are compliant. However there is generally no reason to cache a resource with content disposition so it seems sensible for Firefox to adopt this behavior.
detaro|5 years ago
https://news.ycombinator.com/item?id=22775479
the_mitsuhiko|5 years ago