diff --git a/build/automation.py.in b/build/automation.py.in
index f6de73301081..6dbadf83fbe4 100644
--- a/build/automation.py.in
+++ b/build/automation.py.in
@@ -245,6 +245,7 @@ user_pref("network.manage-offline-status", false);
 user_pref("security.default_personal_cert", "Select Automatically"); // Need to client auth test be w/o any dialogs
 user_pref("network.http.prompt-temp-redirect", false);
 user_pref("svg.smil.enabled", true); // Needed for SMIL mochitests until bug 482402 lands
+user_pref("media.cache_size", 100);
 
 user_pref("camino.warn_when_closing", false); // Camino-only, harmless to others
 """
diff --git a/content/media/video/test/Makefile.in b/content/media/video/test/Makefile.in
index 7841db8d2a35..adb73a69953b 100644
--- a/content/media/video/test/Makefile.in
+++ b/content/media/video/test/Makefile.in
@@ -56,6 +56,7 @@ _TEST_FILES = \
 		test_readyState.html \
 		test_seek2.html \
 		test_volume.html \
+		use_large_cache.js \
 		$(NULL)
 
 ifdef MOZ_OGG
diff --git a/content/media/video/test/test_progress1.html b/content/media/video/test/test_progress1.html
index fcb2d84ccbd7..e8fe1f179d7d 100644
--- a/content/media/video/test/test_progress1.html
+++ b/content/media/video/test/test_progress1.html
@@ -8,6 +8,7 @@
 </head>
 <body>
 <pre id="test">
+<script src="use_large_cache.js"></script>
 <script class="testbody" type="text/javascript">
 var completed = false;
 var load_count = 0;
diff --git a/content/media/video/test/test_progress2.html b/content/media/video/test/test_progress2.html
index 8919d163a74f..68bcb892792d 100644
--- a/content/media/video/test/test_progress2.html
+++ b/content/media/video/test/test_progress2.html
@@ -8,6 +8,7 @@
 </head>
 <body>
 <pre id="test">
+<script src="use_large_cache.js"></script>
 <script class="testbody" type="text/javascript">
 // Test progress events with wav backend
 var completed = false;
diff --git a/content/media/video/test/test_progress3.html b/content/media/video/test/test_progress3.html
index 570ba329fe4b..8ef4d0b88d16 100644
--- a/content/media/video/test/test_progress3.html
+++ b/content/media/video/test/test_progress3.html
@@ -8,6 +8,7 @@
 </head>
 <body>
 <pre id="test">
+<script src="use_large_cache.js"></script>
 <script class="testbody" type="text/javascript">
 // Same as test_progress1 but uses a smaller file to test resource loaded
 // before metadata loaded is fired.
diff --git a/content/media/video/test/test_progress4.html b/content/media/video/test/test_progress4.html
index 861d21ae561d..8d712905a89c 100644
--- a/content/media/video/test/test_progress4.html
+++ b/content/media/video/test/test_progress4.html
@@ -8,6 +8,7 @@
 </head>
 <body>
 <pre id="test">
+<script src="use_large_cache.js"></script>
 <script class="testbody" type="text/javascript">
 // Test progress events with wav backend
 // Same as test_progress2 but uses a smaller file to test resource loaded
diff --git a/content/media/video/test/use_large_cache.js b/content/media/video/test/use_large_cache.js
new file mode 100644
index 000000000000..1b53240c114d
--- /dev/null
+++ b/content/media/video/test/use_large_cache.js
@@ -0,0 +1,14 @@
+(function() {
+  netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
+  // Set cache size to something large
+  var prefService = Components.classes["@mozilla.org/preferences-service;1"]
+                               .getService(Components.interfaces.nsIPrefService);
+  var branch = prefService.getBranch("media.");
+  var oldSize = branch.getIntPref("cache_size");
+  branch.setIntPref("cache_size", 40000);
+
+  window.addEventListener("unload", function() {
+    netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
+    branch.setIntPref("cache_size", oldSize);
+  }, false);
+ })();
