Merge pull request #793 from MITx/bug/tomg/custom-tab-ff

fixed custom tab box-sizing glitch in firefox

lms/static/sass/course/_tabs.scss

 div.static_tab_wrapper {
-
+	@include box-sizing(border-box);
   padding: 2em 2.5em;
 
   h1 {
@@ -9,5 +9,4 @@ div.static_tab_wrapper {
   section {
     margin: 0px 0px 20px;
   }
-
-}
+}
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