--- a/toolkit/content/tests/mochitest/test_mousecapture.xhtml
+++ b/toolkit/content/tests/mochitest/test_mousecapture.xhtml
@@ -71,20 +71,26 @@ function selectionScrollCheck()
     if (count++ < 6)
       return;
 
     otherWindow.removeEventListener("scroll", selectionScrollDone, false);
 
     var selectedText = otherWindow.getSelection().toString().replace(/\r/g, "");
     is(selectedText, "One\n\nTwo", "text is selected");
 
-    // should have scrolled 20 pixels from the mousemove above and six extra
-    // times from the selection scroll timer for a total of 140
-    var oldScrollY = otherWindow.scrollY;
-    is(otherWindow.scrollY, 140, "selection scroll position after timer");
+    // should have scrolled 20 pixels from the mousemove above and at least 6
+    // extra 20-pixel increments from the selection scroll timer. "At least 6"
+    // because we waited for 6 scroll events but multiple scrolls could get
+    // coalesced into a single scroll event. Therefore we allow 6-15 scrolls,
+    // which corresponds to a scroll position of 140 to 320. The 15 is chosen
+    // arbitrarily and can be adjusted.
+    var scrollY = otherWindow.scrollY;
+    ok(scrollY >= 140, "selection scroll position after timer is at least 140");
+    ok(scrollY <= 320, "selection scroll position after timer is not more than 320");
+    ok((scrollY % 20) == 0, "selection scroll position after timer is multiple of 20");
 
     synthesizeMouse(element, 4, otherWindow.innerHeight + 25, { type: "mouseup" }, otherWindow);
     disableNonTestMouseEvents(false);
     otherWindow.close();
 
     if (loadInWindow) {
       SimpleTest.finish();
     }