There is a rough version [0] based on the sidebarAction API [1], this is probably going to improved when it actually lands in Firefox Nightly, which should hope

There is a rough version [0] based on the sidebarAction API [1], this is probably going to improved when it actually lands in Firefox Nightly, which should hopefully be soon. [0] https://github.com/bwinton/TabCenter/tree/webext https://github.com/bwinton/TabCenter/tree/webext [1] https://bugzilla.mozilla.org/show_bug.cgi?id=1208596 https://bugzilla.mozilla.org/show_bug.cgi?id=1208596