Will Firefox support vertical tabs?
Use the hotkey (Ctrl+Shift+V), the VTR icon somewhere in the right top corner (via default) or Firefox's default sidebar icon and switch in the sidebar header's dropdown menu to Vertical Tabs Reloaded. Features: arranges tabs vertical.
Which browsers have vertical tabs?
Microsoft has added vertical tabs as a native feature to its Edge browser, so you'll need no extensions if you use this app to surf the web.