You can customize the Firefox UI entirely by making modifications to userChrome.CSS.
Yeah, but that’s too complicated, Firefox needs better customization options built into the actual browser itself. Something I can just open the options and change how it looks with a few clicks.
And there are some really good addons for tab management in Firefox.
I’ve tried using a few, the only one that even came close to anything in Vivaldi was Sideberry and even that just felt like an inferior version of what Vivaldi natively has.