--- a/browser/themes/osx/browser.css
+++ b/browser/themes/osx/browser.css
@@ -84,18 +84,22 @@
 
 /* These would be margin-inline-start/end if it wasn't for the fact that OS X
  * doesn't reverse the order of the items in the titlebar in RTL mode. */
 .titlebar-placeholder[type="caption-buttons"],
 #titlebar-buttonbox {
   margin-left: 7px;
 }
 
-.titlebar-placeholder[type="fullscreen-button"] {
-  margin-right: 4px;
+/* The fullscreen button doesnt show on Yosemite(10.10) or above so dont give it a
+   border there */
+@media not all and (-moz-mac-yosemite-theme) {
+  .titlebar-placeholder[type="fullscreen-button"] {
+    margin-right: 4px;
+  }
 }
 
 #main-window:not(:-moz-lwtheme) > #titlebar {
   -moz-appearance: -moz-window-titlebar;
 }
 
 #main-window:not([tabsintitlebar]) > #titlebar {
   -moz-appearance: -moz-window-titlebar;