SPDY 3.1 is gone in ESR 52: don't set the pref anymore
SPDY 3.1 is disabled in Firefox 50 (https://bugzilla.mozilla.org/show_bug.cgi?id=1287132) and ripped out in Firefox 51 (https://bugzilla.mozilla.org/show_bug.cgi?id=1248197). No need to set the respective pref anymore.