[wittekm]

They want to add hooks for it, noting Opera already has a sidebar API. https://wiki.mozilla.org/WebExtensions/Future

I've seen similar sentiments echoed by Mozilla devs in a reddit AMA a few weeks ago.

[kuschku]

Sadly, they won’t add nearly enough APIs, they already announced that most of the UI won’t be customizable.

I’d have preferred a full UI customization like in Vivaldi.

[mmebane]

That's disappointing. Do you have a reference for this? I'm curious if there were any details in the announcement.