User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:44.0) Gecko/20100101 Firefox/44.0 Build ID: 20160210153822 Steps to reproduce: Open the Developer Tools and go to the Network tab. Visit any web page that will send a 304 with no content body in response to a request with If-Modified-Since header for cached content. E.g. load this page, then refresh it. Actual results: For any 304 response, the transferred column gives the size of the cached content, not the actual response. Similarly, checking the Response tab shows the cached content, not an empty response body as would be expected. This makes it look like the server is sending a 304 header, but also sending all the content again. If the server does send content with the 304, this is not shown as the response and the cached content is shown as the response. Expected results: The transferred size should be the actual size of the response. The response tab should show what the actual response body was.

Screenshot of Developer tools Network tab showing incorrect information for a 304 response.