From FDE, I disliked the new tabs at first, and I still do. They have no redeeming qualities.
They’re not denser, they’re not more readable, and they’re definitely not more usable as they’re floating in the tab bar unmoored from the rest of the browser. They don’t make any sense and they’re visually unappealing.
I still hate the new tabs after having used the Developer Edition for a while. There's absolutely no separators between tabs, making it hard to see tab boundaries. Having the tabs on a separate background colour also helps to quickly see which part of the chrome is the tab bar and which is the address bar. Now they're the same colour.
FWIW, Chrome has separators between tabs as well as better contrast between the address bar section and the tab section (1.31:1 for Chrome, 1.08:1 for Firefox).
They’re not denser, they’re not more readable, and they’re definitely not more usable as they’re floating in the tab bar unmoored from the rest of the browser. They don’t make any sense and they’re visually unappealing.