What does "faster JS" actually get me? Youtube is probably the most heavy site I and I think most people use, I'm certainly not trying to do heavy scientific computation in my browser, so what difference does it really make?

Anyway, Firefox's killer feature is still extensions, despite everything that's happened on that front. There's nothing like Tree Style Tabs for Chrome (not usably implemented anyway) and while I think maybe Brave has it, Firefox has uMatrix which is better than anything Brave uses (Brave may share lists or even code with that, but the uMatrix UI is where its at.)