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_; }
diff -up firefox-55.0/js/src/jit/ExecutableAllocator.h.old firefox-55.0/js/src/jit/ExecutableAllocator.h
--- firefox-55.0/js/src/jit/ExecutableAllocator.h.old	2017-08-09 09:24:18.784983505 +0200
+++ firefox-55.0/js/src/jit/ExecutableAllocator.h	2017-08-09 09:28:01.471100075 +0200
@@ -307,6 +307,10 @@ class ExecutableAllocator
     {
         sync_instruction_memory((caddr_t)code, size);
     }
+#else
+    static void cacheFlush(void*, size_t)
+    {
+    }
 #endif
 
   private: