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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.

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.

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