(Alternate title: Kaspersky is one typo away from being called Kaspesky)
Bug 1271875 is an interesting case of a compat issue not caused by a website, or a browser, but by a 3rd party. In this case, Kaspersky AntiVirus.
Apparently they Malcolm in the Middle you to keep you safe:
I guess that's normal for Anti-Virus programs?
(Personally I stay safe via a combination of essential oils and hyper-link homeopathy.)
Kaspersky's MITM happily sends
Accept-Encoding: br in the request but strips
Content-Encoding: br from the response. Suddenly Facebook looks like ISIS is trying to hack you:
So, in this instance, if Facebook.com looks like binary garbage in your Firefox (and you have Kaspersky AV installed), consider a new anti-virus strategy (ideally they'll also have an update very soon if you're somehow stuck with it).