Firefox only (Win/Mac/Linux): Firefox extension HideTab minimises open tabs into an icon on the status bar—and even adds a shortcut key to instantly hide everything.
Installing the extension works a little different than normal—you'll need to download the extension and drag the file into the Add-ons window manually. Once you've restarted your browser, head into the options panel and change the default "Panic page" to use something other than the default, and then you can start hiding tabs through the context menu or using the handy Ctrl+Q shortcut key. To hide all tabs instead of just the current one, use the Alt+Q shortcut key instead, and Ctrl+Alt+Q will restore all hidden tabs, though you can pick and choose from the icon in the status bar.
HideTab is a free download, works wherever Firefox does. For a similar extension that closes all your open tabs instead of just hiding them, check out previously mentioned Panic, or learn how to hide your inappropriate computing with Magic Boss Key.