Bug 1111091 - Move tab separator to the start of the last tab with separated tab button;r=MattN

browser/themes/shared/tabs.inc.css

@@ -316,6 +316,14 @@
   width: 3px;
 }
 
+/* Handle a case where the last separator in a customized tab bar with a
+   separated new tab button causes the width of the tab to extend and causes
+   jittering with a small tab curve width (Bug 1111091). */
+#TabsToolbar[currentset]:not([currentset*="tabbrowser-tabs,new-tab-button"]) #tabbrowser-tabs:not([overflow]) > .tabbrowser-tab[last-visible-tab]:not([selected]):not([beforehovered]):not(:hover)::after {
+  -moz-margin-start: -3px;
+  -moz-margin-end: 0;
+}
+
 /* New tab button */
 
 .tabs-newtab-button {