In bug 1452645 [1] we are planning to remove the "Load this bookmark in
sidebar" feature. As part of this we will also be removing support for
rel="sidebar" in anchor elements.
In the future, anchor elements with rel="sidebar" will be treated like
normal anchor elements.
We are planning to make the change in Firefox 63.
From the specifications side, rel="sidebar" is only implemented by
Firefox [2] and although it seems to have been mentioned in early
versions of the HTML 5 specification, it isn't in the final version.
Extensions are able to open pages in the sidebar [3] (e.g. side view
[4]), and intercept loads (e.g. see the Facebook Container add-on [5]),
hence, they could add very similar functionality.
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1452645
[2] https://developer.mozilla.org/docs/Web/HTML/Link_types
[3]
https://developer.mozilla.org/Add-ons/WebExtensions/API/sidebarAction/setPanel
[4] https://testpilot.firefox.com/experiments/side-view
[5] https://addons.mozilla.org/firefox/addon/facebook-container/
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