--- a/toolkit/components/thumbnails/test/thumbnails_background.sjs
+++ b/toolkit/components/thumbnails/test/thumbnails_background.sjs
@@ -3,17 +3,17 @@
 
 // The timer never fires if it's not declared and set to this variable outside
 // handleRequest, as if it's getting GC'ed when handleRequest's scope goes away.
 // Shouldn't the timer thread hold a strong reference to it?
 var timer;
 
 function handleRequest(req, resp) {
   resp.processAsync();
-  resp.setHeader("Cache-Control", "no-cache", false);
+  resp.setHeader("Cache-Control", "no-cache, no-store", false);
   resp.setHeader("Content-Type", "text/html;charset=utf-8", false);
 
   let opts = {};
   try {
     opts = JSON.parse(decodeURIComponent(req.queryString));
   }
   catch (err) {}