I found no setting. Im using an addon "ModHeader" to set the response header (for all websites) "Expires" to a date far in the future. It worked in the past, but doesn't work at the moment. Have to debug it.

Wouldn't you need to modify Cache-Control?

>>If there is a Cache-Control header with the max-age or s-maxage directive in the response, the Expires header is ignored.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Ex...

Edit: Also I'm not sure that is the relevant one? Reddit sets Cache-Control to "private, max-age=3600 [seconds]", but it definitely reloads the content if I back up in less than an hour.