User-Agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:2.0.1) Gecko/20100101 Firefox/4.0.1 Build Identifier: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:2.0.1) Gecko/20100101 Firefox/4.0.1 In the context of extension development it is often needed to listen to the closing of the Firefox sidebar. An easy way to do so is to listen on events on the sidebar close button. Alas this button has no "id", thus one has to use a query selector to get this XUL element. Reproducible: Always Actual Results: var sidebar_closebutton = top.document.querySelector('#sidebar-box toolbarbutton'); sidebar_closebutton.addEventListener('command', reactToClosing, false); Expected Results: If the sidebar close button XUL element could have an ID such as "sidebar-closebutton" extension developers could add listeners to it in a much more robust manner without relying on somehow implementation details like that: var sidebar_closebutton = top.document.getElementById('sidebar-closebutton'); sidebar_closebutton.addEventListener('command', reactToClosing, false); As you see this is just a suggestion of a small addition that is fully compatible with both past and future. Thanks a lot if this little id attribute could be added!