One of the best things that happened to FireFox (IMO).
There are few quirks but I circumvented them by disabling cache, drag of tree tabs to another window and most important by shutting down TST sidebar before closing browser (that last part prevents from "mangling" order of tabs inside tree at launch).
Thanks to that I can safely manage 2000+ tabs on small Atom based CPU.

P.S. Please remember that one can hide FireFox native tab bar at top by editing profile file userChrome.css and adding lines:

/* to hide the native tabs */
#TabsToolbar {
visibility: collapse;
}

/* to hide the sidebar header */
#sidebar-header {
visibility: collapse;
}