Note: if you're upgrading Firefox and you didn't have SPDY enabled already you'll have to enable it manually.
(about:config, network.http.spdy.enabled = true)
(about:config, network.http.spdy.enabled = true)
[Note: I am a Firefox developer.]
[Note: I literally just downloaded it, upgraded, and had this experience.]
The bug is 8 years old and nobody seems to be working on it. I wouldn't hold my breath that it will be fixed soon.