I just gave it another try. Unfortunately, things haven't improved. `about:performance` shows a "medium" energy impact for TST. My 2015 Macbook's fans come on and stay on until I disable TST again, after which they reliably spool back down after a short delay. While TST is enabled, Firefox shoots up the list of CPU users in Activity Monitor.

Oh well, it was worth a shot, sidebar tabs are just so good when they're working well, but it's not worth the battery drain.

Your userChrome.css tweaks worked great!