--- a/js/src/ion/MIR.cpp
+++ b/js/src/ion/MIR.cpp
@@ -854,17 +854,24 @@ MBitNot::infer()
         specialization_ = MIRType_None;
     else
         specialization_ = MIRType_Int32;
 }
 
 static inline bool
 IsConstant(MDefinition *def, double v)
 {
-    return def->isConstant() && def->toConstant()->value().toNumber() == v;
+    if (!def->isConstant())
+        return false;
+
+    // Compare as bits to avoid conflating, e.g., -0 and 0.
+    mozilla::detail::DoublePun lhs, rhs;
+    lhs.d = def->toConstant()->value().toNumber();
+    rhs.d = v;
+    return lhs.u == rhs.u;
 }
 
 MDefinition *
 MBinaryBitwiseInstruction::foldsTo(bool useValueNumbers)
 {
     if (specialization_ != MIRType_Int32)
         return this;
 

new file mode 100644
--- /dev/null
+++ b/js/src/jit-test/tests/basic/testBug878429.js
@@ -0,0 +1,9 @@
+function negZeroMinusNegZero()
+{
+  var x = -0.0;
+  var y = -0.0;
+  return +(x - y);
+}
+
+assertEq(1 / negZeroMinusNegZero(), Infinity);
+assertEq(1 / negZeroMinusNegZero(), Infinity);