They want to add hooks for it, noting Opera already has a sidebar  API.
<a href="https:&#x2F;&#x2F;wiki.mozilla.org&#x2F;WebExtensions&#x2F;Future" rel="nofollow">https:&#x2F;&#x2F;wiki.mozilla.org&#x2F;WebExtensions&#x2F;Future</a><p>I&#x27;ve seen similar sentiments echoed by Mozilla devs in a reddit AMA a few weeks ago.