Firefox 63 has removed the support for the
sidebar link type that allowed users to bookmark a link that would later be opened in the browser’s sidebar. The
window.sidebar.addPanel method to do the same thing has already been dropped with Firefox 23.
<a rel="sidebar"> will be treated like normal anchor elements from now on. Given that the functionality has been implemented only in Firefox, the compatibility risk should be very low. If your website wants to offer visitors a sidebar panel for easier access, you can create a browser extension instead.