diff -up firefox-55.0/js/src/jit/MIR.h.old firefox-55.0/js/src/jit/MIR.h
--- firefox-55.0/js/src/jit/MIR.h.old	2017-08-08 14:04:44.528460099 +0200
+++ firefox-55.0/js/src/jit/MIR.h	2017-08-08 14:05:11.045364831 +0200
@@ -12434,7 +12434,7 @@ class MNearbyInt
     TRIVIAL_NEW_WRAPPERS
 
     static bool HasAssemblerSupport(RoundingMode mode) {
-        return Assembler::HasRoundInstruction(mode);
+        return false;
     }
 
     RoundingMode roundingMode() const { return roundingMode_; }