Firefox 44+ accepts a new kind of encoding (compression) called Brotli (br) for secure connections and Facebook turned on Brotli encoding last night. Kapsersky doesn't know about this encoding and strips it from the HTTP response headers if you have "Inject script" checked in Kapsersky Settings -> Additional -> Network -> Traffic Processing https://forum.kaspersky.com/index.php?s=bbdfcf797a91bc90230472c52c4b103e&showtopic=351027 ''originally posted by James in another thread''