Mozilla Connect

Oh I see something similar is voted here:https://connect.mozilla.org/t5/ideas/native-vertical-tabs/idi-p/85/page/20#comments

You're not alone. TRhere's a whole Community around vertical, auto-or button hidden Tabbars:https://bugzilla.mozilla.org/show_bug.cgi?id=1332447https://www.reddit.com/r/FirefoxCSS/comments/oljbzr/toggle_tabbar_visibility_with_alt_continued/