MultiSelect Tabs by mike_conley

Experimental

This add-on is no longer maintained! Firefox 64 is introducing built-in multi-select tabs, so just use those instead. Source: <a rel="nofollow" href="https://prod.outgoing.prod.webservices.mozgcp.net/v1/df5daf949c7c1f9b8824c241f8846486f43ac34b42fa454ff34567f9f03ad889/https%3A//github.com/mikeconley/multi-select-tabs">https://github.com/mikeconley/multi-select-tabs</a>

You'll need Firefox to use this extension