diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index f0cc3c40f0..c9f70cae4d 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -3642,14 +3642,22 @@ GCRuntime::beginMarkPhase(JS::gcreason::Reason reason, AutoLockForExclusiveAcces
     }
 
     /*
-     * Atoms are not in the cross-compartment map. So if there are any
-     * zones that are not being collected, we are not allowed to collect
-     * atoms. Otherwise, the non-collected zones could contain pointers
-     * to atoms that we would miss.
+     * Atoms are not in the cross-compartment map. If there are any zones that
+     * are not being collected then we cannot collect the atoms zone, otherwise
+     * the non-collected zones could contain pointers to atoms that we would
+     * miss.
      *
-     * keepAtoms() will only change on the main thread, which we are currently
-     * on. If the value of keepAtoms() changes between GC slices, then we'll
-     * cancel the incremental GC. See IsIncrementalGCSafe.
+     * If keepAtoms() is true then either an instance of AutoKeepAtoms is
+     * currently on the stack or parsing is currently happening on another
+     * thread. In either case we don't have information about which atoms are
+     * roots, so we must skip collecting atoms.
+     *
+     * Note that only affects the first slice of an incremental GC since root
+     * marking is completed before we return to the mutator.
+     *
+     * Off-main-thread parsing is inhibited after the start of GC which prevents
+     * races between creating atoms during parsing and sweeping atoms on the
+     * main thread.
      */
 	 
     if (isFull && !rt->keepAtoms()) {
@@ -5732,6 +5740,10 @@ GCRuntime::gcCycle(bool nonincrementalByAPI, SliceBudget& budget, JS::gcreason::
 
     State prevState = incrementalState;
 
+    // We don't allow off-main-thread parsing to start while we're doing an
+    // incremental GC.
+    MOZ_ASSERT_IF(rt->activeGCInAtomsZone(), !rt->exclusiveThreadsPresent());
+
     if (nonincrementalByAPI) {
         // Reset any in progress incremental GC if this was triggered via the
         // API. This isn't required for correctness, but sometimes during tests
diff --git a/js/src/jsgc.h b/js/src/jsgc.h
index 521dea05c6..da3de0930d 100644
--- a/js/src/jsgc.h
+++ b/js/src/jsgc.h
@@ -56,7 +56,7 @@ enum class State {
     D(None) \
     D(NonIncrementalRequested) \
     D(AbortRequested) \
-    D(KeepAtomsSet) \
+    D(Unused1) \
     D(IncrementalDisabled) \
     D(ModeChange) \
     D(MallocBytesTrigger) \