--- a/dom/base/test/browser_messagemanager_loadprocessscript.js
+++ b/dom/base/test/browser_messagemanager_loadprocessscript.js
@@ -1,12 +1,26 @@
 var ppmm = Cc["@mozilla.org/parentprocessmessagemanager;1"]
            .getService(Ci.nsIMessageBroadcaster);
 ppmm.QueryInterface(Ci.nsIProcessScriptLoader);
 
+const BASE_NUMBER_OF_PROCESSES = 3;
+function checkBaseProcessCount(description) {
+  const {childCount} = ppmm;
+  // With preloaded activity-stream, process count is a bit undeterministic, so
+  // allow for some variation
+  if (Services.prefs.getBoolPref("browser.newtabpage.activity-stream.enabled")) {
+    const extraCount = BASE_NUMBER_OF_PROCESSES + 1;
+    ok(childCount === BASE_NUMBER_OF_PROCESSES || childCount === extraCount, `${description} (${BASE_NUMBER_OF_PROCESSES} or ${extraCount})`);
+  }
+  else {
+    is(childCount, BASE_NUMBER_OF_PROCESSES, `${description} (${BASE_NUMBER_OF_PROCESSES})`);
+  }
+}
+
 function processScript() {
   let cpmm = Components.classes["@mozilla.org/childprocessmessagemanager;1"]
            .getService(Components.interfaces.nsISyncMessageSender);
   if (cpmm !== this) {
     dump("Test failed: wrong global object\n");
     return;
   }
 
@@ -71,17 +85,17 @@ add_task(async function(){
     tabs[i] = await BrowserTestUtils.openNewForegroundTab(gBrowser, "about:blank");
   }
 
   for (let i = 0; i < 3; i++) {
     await BrowserTestUtils.removeTab(tabs[i]);
   }
 
   ppmm.releaseCachedProcesses();
-  is(ppmm.childCount, 3, "Should get back to 3 processes at this point.");
+  checkBaseProcessCount("Should get back to the base number of processes at this point");
 })
 
 // Test that loading a process script loads in all existing processes
 add_task(async function() {
   let checks = [];
   for (let i = 0; i < ppmm.childCount; i++)
     checks.push(checkProcess(ppmm.getChildAt(i)));
 
@@ -90,17 +104,17 @@ add_task(async function() {
 });
 
 // Test that loading a process script loads in new processes
 add_task(async function() {
   // This test is only relevant in e10s
   if (!gMultiProcessBrowser)
     return;
 
-  is(ppmm.childCount, 3, "Should be three processes at this point");
+  checkBaseProcessCount("Should still be at the base number of processes at this point");
 
   // Load something in the main process
   gBrowser.selectedBrowser.loadURI("about:robots");
   await BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
 
   let init = ppmm.initialProcessData;
   init.test123 = "hello";
   init.test456 = new Map();
@@ -120,17 +134,17 @@ add_task(async function() {
     await check;
 
     check = checkProcess(ppmm);
     // Reset the default browser to start a new child process
     gBrowser.updateBrowserRemoteness(gBrowser.selectedBrowser, true);
     gBrowser.selectedBrowser.loadURI("about:blank");
     await BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
 
-    is(ppmm.childCount, 3, "Should be back to three processes at this point");
+    checkBaseProcessCount("Should be back to the base number of processes at this point");
 
     // The new process should have responded
     await check;
 
     ppmm.removeDelayedProcessScript(processScriptURL);
 
     let childMM;
     childMM = ppmm.getChildAt(2);