diff --git a/services/common/tests/unit/test_hawkrequest.js b/services/common/tests/unit/test_hawkrequest.js
index ac42d5a7bb3c..3b7d77c7d619 100644
--- a/services/common/tests/unit/test_hawkrequest.js
+++ b/services/common/tests/unit/test_hawkrequest.js
@@ -43,7 +43,7 @@ add_test(function test_intl_accept_language() {
     "fa-CG;ik",     // Congolese dialect of Farsei, defaulting to Inupiaq
   ];
 
-  function setLanguage(lang) {
+  function setLanguagePref(lang) {
     let acceptLanguage = Cc["@mozilla.org/supports-string;1"]
                            .createInstance(Ci.nsISupportsString);
     acceptLanguage.data = lang;
@@ -53,20 +53,27 @@ add_test(function test_intl_accept_language() {
 
   let hawk = new HAWKAuthenticatedRESTRequest("https://example.com");
 
-  Services.prefs.addObserver("intl.accept_languages", nextTest, false);
-  setLanguage(languages[testCount]);
+  Services.prefs.addObserver("intl.accept_languages", checkLanguagePref, false);
+  setLanguagePref(languages[testCount]);
 
-  function nextTest() {
+  function checkLanguagePref() {
+    var _done = false;
     CommonUtils.nextTick(function() {
-      if (testCount < 2) {
-        do_check_eq(hawk._intl.accept_languages, languages[testCount]);
+      // Ensure we're only called for the number of entries in languages[].
+      do_check_true(testCount < languages.length);
 
-        testCount += 1;
-        setLanguage(languages[testCount]);
-        nextTest();
+      do_check_eq(hawk._intl.accept_languages, languages[testCount]);
+
+      testCount++;
+      if (testCount < languages.length) {
+        // Set next language in prefs; Pref service will call checkNextLanguage.
+        setLanguagePref(languages[testCount]);
         return;
       }
-      Services.prefs.removeObserver("intl.accept_languages", nextTest);
+
+      // We've checked all the entries in languages[]. Cleanup and move on.
+      do_print("Checked " + testCount + " languages. Removing checkLanguagePref as pref observer.");
+      Services.prefs.removeObserver("intl.accept_languages", checkLanguagePref);
       run_next_test();
       return;
     });