There are a number of ways to quickly open Gmail's compose window, but reader germcd shares a handy tip that works with any mail client.
All it takes is a quick command in your address bar:
mailto:into the address bar opens an empty compose window for me.
And, if you have a different program (like Thunderbird or Postbox) set as your default mail client, it'll open a compose window in those programs instead. Pretty handy! Just like our awesome custom searches you should enable in your browser, this is a quick shortcut that just requires a few keystrokes to complete (especially if you use Ctrl+L to focus the address bar). At least it's quicker than clicking on your email app!