One feature that has been in Safari for a long time which I wish came to other browsers was the Gallery view for open tabs.
It displays open tabs as a grid of thumbnails. As a heavy tab user, it would be far easier to sort through (and close) open tabs than going through one by one as I have to currently in Firefox.
Tab Groups / Containers could also easily be integrated in to this as well (potentially allowing drag and drop to reorder and sort tabs between containers etc.
Unfortunately I haven't even found any addons that provide this but I would absolutely love for this to become a part of Firefox.