--- a/js/public/SliceBudget.h
+++ b/js/public/SliceBudget.h
@@ -33,17 +33,17 @@ struct JS_PUBLIC_API(WorkBudget)
  */
 struct JS_PUBLIC_API(SliceBudget)
 {
     int64_t deadline; /* in microseconds */
     intptr_t counter;
 
     static const intptr_t CounterReset = 1000;
 
-    static const int64_t Unlimited = 0;
+    static const int64_t Unlimited = -1;
 
     /* Use to create an unlimited budget. */
     SliceBudget();
 
     /* Instantiate as SliceBudget(TimeBudget(n)). */
     explicit SliceBudget(TimeBudget time);
 
     /* Instantiate as SliceBudget(WorkBudget(n)). */
@@ -54,17 +54,17 @@ struct JS_PUBLIC_API(SliceBudget)
         counter = unlimitedStartCounter;
     }
 
     void step(intptr_t amt = 1) {
         counter -= amt;
     }
 
     bool isOverBudget() {
-        if (counter >= 0)
+        if (counter > 0)
             return false;
         return checkOverBudget();
     }
 
     bool isUnlimited() {
         return deadline == unlimitedDeadline;
     }
 

--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -1452,17 +1452,17 @@ js::gc::FinishPersistentRootedChains(JSR
 void
 GCRuntime::setParameter(JSGCParamKey key, uint32_t value)
 {
     switch (key) {
       case JSGC_MAX_MALLOC_BYTES:
         setMaxMallocBytes(value);
         break;
       case JSGC_SLICE_TIME_BUDGET:
-        sliceBudget = value;
+        sliceBudget = value ? value : SliceBudget::Unlimited;
         break;
       case JSGC_MARK_STACK_LIMIT:
         setMarkStackLimit(value);
         break;
       case JSGC_DECOMMIT_THRESHOLD:
         decommitThreshold = value * 1024 * 1024;
         break;
       case JSGC_MODE:
@@ -2932,34 +2932,39 @@ GCRuntime::refillFreeListInGC(Zone *zone
 
 SliceBudget::SliceBudget()
 {
     reset();
 }
 
 SliceBudget::SliceBudget(TimeBudget time)
 {
-    if (time.budget == Unlimited) {
+    if (time.budget < 0) {
         reset();
     } else {
+        // Note: TimeBudget(0) is equivalent to WorkBudget(CounterReset).
         deadline = PRMJ_Now() + time.budget * PRMJ_USEC_PER_MSEC;
         counter = CounterReset;
     }
 }
 
 SliceBudget::SliceBudget(WorkBudget work)
 {
-    deadline = 0;
-    counter = work.budget;
+    if (work.budget < 0) {
+        reset();
+    } else {
+        deadline = 0;
+        counter = work.budget;
+    }
 }
 
 bool
 SliceBudget::checkOverBudget()
 {
-    bool over = PRMJ_Now() > deadline;
+    bool over = PRMJ_Now() >= deadline;
     if (!over)
         counter = CounterReset;
     return over;
 }
 
 void
 js::MarkCompartmentActive(InterpreterFrame *fp)
 {