Here's the relevant Gecko (Firefox) code which tries to use the max-age and expires headers first and then will set it to forever if the response code was 300,

Here's the relevant Gecko (Firefox) code which tries to use the max-age and expires headers first and then will set it to forever if the response code was 300, 410, 301, or 308. Note that I'm going by a somewhat shallow code reading after a recent investigation. There's a lot of stuff going on in gecko/necko that could potentially apply some failsafe time limit, maybe in the cache implementation, so I wouldn't take this as 100% for sure. Breaking on this function with gdb and tracing the flow is probably a better idea if you really want to know. https://dxr.mozilla.org/mozilla-central/source/netwerk/protocol/http/nsHttpResponseHead.cpp#411 https://dxr.mozilla.org/mozilla-central/source/netwerk/proto... The 301, 308 stuff comes from IsPermanentRedirect which is here: https://dxr.mozilla.org/mozilla-central/source/netwerk/protocol/http/nsHttpResponseHead.cpp#411 https://dxr.mozilla.org/mozilla-central/source/netwerk/proto...