Start Firefox in the Profile selector and try using a fresh new profile. Or start in safe mode. Some old, forgotten configuration or addons are sometimes the ca

Start Firefox in the Profile selector and try using a fresh new profile. Or start in safe mode. Some old, forgotten configuration or addons are sometimes the cause. For example a privacy-enhancing addon CleanURLs disables ETag functionality by default. So all sites using ETag for caching content won't be cached. Big loss.