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diff --git a/js/xpconnect/tests/unit/test_onGarbageCollection-04.js b/js/xpconnect/tests/unit/test_onGarbageCollection-04.js
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+++ b/js/xpconnect/tests/unit/test_onGarbageCollection-04.js
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+// Test that the onGarbageCollection reentrancy guard is on a per Debugger
+// basis. That is if our first Debugger is observing our second Debugger's
+// compartment, and this second Debugger triggers a GC inside its
+// onGarbageCollection hook, the first Debugger's onGarbageCollection hook is
+// still called.
+//
+// This is the scenario we are setting up: top level debugging the `debuggeree`
+// global, which is debugging the `debuggee` global. Then, we trigger the
+// following events:
+//
+//     debuggee gc
+//         |
+//         V
+//     debuggeree's onGarbageCollection
+//         |
+//         V
+//     debuggeree gc
+//         |
+//         V
+//     top level onGarbageCollection
+//
+// Note that the top level's onGarbageCollection hook should be fired, at the
+// same time that we are preventing reentrancy into debuggeree's
+// onGarbageCollection hook.
+
+function run_test() {
+  do_test_pending();
+
+  const debuggeree = newGlobal();
+  const debuggee = debuggeree.debuggee = newGlobal();
+
+  debuggeree.eval(
+    `
+    var dbg = new Debugger(this.debuggee);
+    var fired = 0;
+    dbg.memory.onGarbageCollection = _ => {
+      fired++;
+      gc(this);
+    };
+    `
+  );
+
+  const dbg = new Debugger(debuggeree);
+  let fired = 0;
+  dbg.memory.onGarbageCollection = _ => {
+    fired++;
+  };
+
+  debuggee.eval(`gc(this)`);
+
+  // Let first onGarbageCollection runnable get run.
+  executeSoon(() => {
+
+    // Let second onGarbageCollection runnable get run.
+    executeSoon(() => {
+
+      // Even though we request GC'ing a single zone, we can't rely on that
+      // behavior and both zones could have been scheduled for gc for both
+      // gc(this) calls.
+      ok(debuggeree.fired >= 1);
+      ok(fired >= 1);
+
+      debuggeree.dbg.enabled = dbg.enabled = false;
+      do_test_finished();
+    });
+  });
+}