You don't need an extension. Right-click on a tab and "Select All Tabs". Right-click again and it has the option "Close 1,122 Tabs". Your number may be smaller.

That only works if you've got a single window open. For myself, I keep ~10+ windows open, with then ~8 tabs per window. Note this is only practical on a tiling window manager. Anyway, the tab count extension may still be the way to go.