Oops, I meant to remove this unneeded pack=end before checking in, per jag's comments.

xpfe/components/filepicker/res/content/filepicker.xul

@@ -91,7 +91,7 @@
     </row>
   </rows>
 </grid>
-<hbox class="dialog-button-box" align="center" pack="end">
+<hbox class="dialog-button-box" align="center">
   <checkbox label="&showHiddenFiles.label;" oncommand="toggleShowHidden();"
             flex="1" align="left"/>
   <button dlgtype="accept" class="dialog-button"/>