Stil buggy, open a link on a child tab and it will hide, despite disabling the option to not collapse the tab, this happens plenty of times, and it's annoying. Rarely happens but, sometimes the tab will just be out of sight, and have to CTRL+TAB to get to it. There should be a way of resizing them, they are too large for my eyes.

Still good tho.

Dev if you need a showcase, or a way to replicate the collapsing issue, go to the Tree Style Tab extension page (https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/) and go to More extensions by Piro (piro_or) and middle click in one of them, make sure there are no other child tabs, otherwise it won't work.