That, plus TreeStyleTab because every monitor is wide enough to get rid of the tabs bar https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...
I hate to say it but the best vertical tab implementation I've seen is in Edge. Easily toggle-able between horizontal/vertical tabs, there's a way to collapse it to just favicons, you can use collections to group tabs together (like in Chrome). It's honestly one of the features that keeps me coming back to Edge, privacy be damned.
I think the only feature I really miss from Firefox is container tabs.
$ cat userChrome.css
/* Hide horizontal tabs at the top of the window */
#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {
opacity: 0;
pointer-events: none;
}
#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
visibility: collapse !important;
}
/* Hide the "Tree Style Tab" header at the top of the sidebar */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
display: none;
}For many users this is TST's main benefit.