[Ringz]

I have no problem with hundreds of tabs on Firefox.

[NoMoreNicksLeft]

There is a "tab count" extension. Install it only if you want to learn some awful truths about yourself.

[masfuerte]

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.

[lelandbatey]

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.