Firefox 44+ accepts a new kind of encoding (compression) called Brotli (br) for secure connections. Facebook has recently enabled Brotli (br) encoding for files send via a secure connections. Some (security) software that intercepts a secure connection to scan the content doesn't know about this encoding and changes the content-type header to text/plain. A possible workaround is to modify the involved pref and remove the trailing ", br" to prevent the server from sending files with Brotli compression. *network.http.accept-encoding.secure = "gzip, deflate, br" => "gzip, deflate" (without quotes) You can open the <b>about:config</b> page via the location/address bar. You can accept the warning and click "I'll be careful" to continue. *http://kb.mozillazine.org/about:config