> The replacement that is compatible with the new form of extension isn't able to unload the tabs, just hide them, which undoes most of the performance benefits.

I believe this information is out of date. Firefox 58 brought extension developers the ability to unload tabs: https://bugzilla.mozilla.org/show_bug.cgi?id=1322485

And Totally Not Panorama takes advantage: https://addons.mozilla.org/en-GB/firefox/addon/basic-panoram...