--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -601,17 +601,17 @@ Tester.prototype = {
         // use a shrinking GC so that the JS engine will discard JIT code and
         // JIT caches more aggressively.
 
         let shutdownCleanup = aCallback => {
           Cu.schedulePreciseShrinkingGC(() => {
             // Run the GC and CC a few times to make sure that as much
             // as possible is freed.
             let numCycles = 3;
-            for (i = 0; i < numCycles; i++) {
+            for (let i = 0; i < numCycles; i++) {
               Cu.forceGC();
               Cu.forceCC();
             }
             aCallback();
           });
         };