Since there are a lot of popular addons which modify the browser looks, I wonder how they will be able to make a transition?
For example all these great addons (https://addons.mozilla.org/en-US/firefox/user/quicksaver/) can not be ported as the developer states.
AFAIK the new theming system will be able to manipulate the UI with CSS, and FF54 supports Opera-style sidebars.
What is missing (and what would be needed by the Findbar and Tab-Groups add-ons) are page overlays and some notion of hidden tabs.
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You will also be able to hide the tab strip soon, there are discussions about a toolbar API and replacing existing UI elements.