Bug 1492582 - Fix test_queryCaretRect.html. r=dao

Differential Revision: https://phabricator.services.mozilla.com/D49469

--HG--
extra : moz-landing-system : lando

browser/themes/osx/browser.css

@@ -60,8 +60,7 @@
   content: "";
   display: -moz-box;
   -moz-appearance: toolbox;
-  height: 1px;
-  margin-top: -1px;
+  height: 0;
   opacity: 0.001;
 }