One of the greatest multiple-tab features is available in the Multiple Tab Handler [0] in combination with Tree-Style Tabs: click-drag to close (or select). It's a bit more tedious in the current non-XUL versions of FF (have to mouse down longer), but basically if you need to close like 50 tabs in your tree, you can mouse down over one "x" and drag selecting the other "x"s and once you mouse up it closes them all. Want to close every other one or a more specific set? With the mouse down, only go over the "x"s you want to close. The same thing on the tab itself for selecting multiple tabs to do anything like reparent, reload, etc.

0 - https://addons.mozilla.org/en-US/firefox/addon/multiple-tab-...