I actually love the tab design, which is odd, because I hate anything new. What I did NOT like is the vertical spacing, which stank of the superfluous header bars from the late 90s/early 2000s.

I applied a browser CSS change to fix it, and now I think it's great.

    /*
     * remove dead space in browser tabs bar
     *
     * Source: https://www.reddit.com/r/firefox/comments/nq6kto/if_you_dont_like_the_new_design_you_can_disable_it/
     */
    :root { --tab-min-height: 30px !important; }
    :root #tabbrowser-tabs { --tab-min-height: 30px !important; }