fixing modal position in FF

wagtail/wagtailadmin/static/wagtailadmin/css/components/modals.less

@@ -48,7 +48,7 @@
 .modal-content {
     .border-box();
     .border-radius(3px);
-    width:99.70%;
+    width:98.70%;
     position: relative;
     background-color: white;
     outline: none;