--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -645,16 +645,18 @@ Tester.prototype = {
           "ShutdownLeaks: Wait for cleanup to be finished before checking for leaks");
         Services.obs.notifyObservers({wrappedJSObject: barrier},
           "shutdown-leaks-before-check");
 
         barrier.client.addBlocker("ShutdownLeaks: Wait for tabs to finish closing",
                                   TabDestroyObserver.wait());
 
         barrier.wait().then(() => {
+          // Close all active HTTP connections.
+          Services.obs.notifyObservers(null, "net:prune-all-connections", null);
           // Simulate memory pressure so that we're forced to free more resources
           // and thus get rid of more false leaks like already terminated workers.
           Services.obs.notifyObservers(null, "memory-pressure", "heap-minimize");
 
           Services.ppmm.broadcastAsyncMessage("browser-test:collect-request");
 
           shutdownCleanup(() => {
             setTimeout(() => {