diff --git a/layout/tools/reftest/reftest-cmdline.js b/layout/tools/reftest/reftest-cmdline.js
index 1078561513ca..8166be21d6b5 100644
--- a/layout/tools/reftest/reftest-cmdline.js
+++ b/layout/tools/reftest/reftest-cmdline.js
@@ -105,6 +105,8 @@ RefTestCmdLineHandler.prototype =
     // Checking whether two files are the same is slow on Windows.
     // Setting this pref makes tests run much faster there.
     branch.setBoolPref("security.fileuri.strict_origin_policy", false);
+    // Disable the background thumbnailing service
+    branch.setBoolPref("browser.pageThumbs.enabled", false);
 
     var wwatch = Components.classes["@mozilla.org/embedcomp/window-watcher;1"]
                            .getService(nsIWindowWatcher);