diff -r 000000000000 -r 6474c204b198 browser/base/content/test/general/browser_bug585785.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/base/content/test/general/browser_bug585785.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,35 @@
+var tab;
+
+function test() {
+  waitForExplicitFinish();
+
+  tab = gBrowser.addTab();
+  isnot(tab.getAttribute("fadein"), "true", "newly opened tab is yet to fade in");
+
+  // Try to remove the tab right before the opening animation's first frame
+  window.mozRequestAnimationFrame(checkAnimationState);
+}
+
+function checkAnimationState() {
+  is(tab.getAttribute("fadein"), "true", "tab opening animation initiated");
+
+  info(window.getComputedStyle(tab).maxWidth);
+  gBrowser.removeTab(tab, { animate: true });
+  if (!tab.parentNode) {
+    ok(true, "tab removed synchronously since the opening animation hasn't moved yet");
+    finish();
+    return;
+  }
+
+  info("tab didn't close immediately, so the tab opening animation must have started moving");
+  info("waiting for the tab to close asynchronously");
+  tab.addEventListener("transitionend", function (event) {
+    if (event.propertyName == "max-width") {
+      tab.removeEventListener("transitionend", arguments.callee, false);
+      executeSoon(function () {
+        ok(!tab.parentNode, "tab removed asynchronously");
+        finish();
+      });
+    }
+  }, false);
+}