diff --git a/js/public/SliceBudget.h b/js/public/SliceBudget.h
index 2dfbdacc73e5..c45fdf020853 100644
--- a/js/public/SliceBudget.h
+++ b/js/public/SliceBudget.h
@@ -38,7 +38,7 @@ struct JS_PUBLIC_API(SliceBudget)
 
     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();
@@ -59,7 +59,7 @@ struct JS_PUBLIC_API(SliceBudget)
     }
 
     bool isOverBudget() {
-        if (counter >= 0)
+        if (counter > 0)
             return false;
         return checkOverBudget();
     }
diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index 116b33e0455b..d4d98806a4a5 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -1457,7 +1457,7 @@ GCRuntime::setParameter(JSGCParamKey key, uint32_t value)
         setMaxMallocBytes(value);
         break;
       case JSGC_SLICE_TIME_BUDGET:
-        sliceBudget = value;
+        sliceBudget = value ? value : SliceBudget::Unlimited;
         break;
       case JSGC_MARK_STACK_LIMIT:
         setMarkStackLimit(value);
@@ -2937,9 +2937,10 @@ SliceBudget::SliceBudget()
 
 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;
     }
@@ -2947,14 +2948,18 @@ SliceBudget::SliceBudget(TimeBudget time)
 
 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;