NOTE THIS EDIT: I just noticed configuring TST and following it's advice of changing configuratin 'browser.tabs.multiselect' to true, but it didn't exist! This is 77.0.1 64-bit Linux version and that option didn't exists - I added it, but as it was not there to begin with, I doubt it will be of any help that I added it... Advice?
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Awesome! I used to love an old legacy add-on called Tab Mix Plus, which really is an awesome add-on for the tab bar, but now I love the tab tree much more - it's also specifically nice with small netbooks, like my Acer Aspire One D270 (old, I know, but still works marvels if not put under too heavy strain), that have a widescreen display and very low vertical resolution - you can hide the sidebar if you need to, although most of the time it's not a problem, and you save the height of the tab bar in vertical resolution available to show the actual page. With such small displays it really does matter a lot.

Edit: Many people are complaining about the lack of search function. I'm not sure what exactly they mean, I'm completely satisfied with being able to search tabs using the location bar - you can enable this from FireFox settings. However maybe a search function that only shows the matching tabs (and the tab-tree they are part of) in the sidebar - when it's empty it would show them all. I can't think of another way they mean, but I'm really fully content with searching for the tab using location bar anyway :)