I fell for the link in Firefox and their popup blocker didn't catch it, even though I'm running the latest version and I have auto-updates turned on. Good thing my speakers were turned off.
If it hits you:
1. It's easy enough to hit ctrl-alt-delete and bring up task manager to close the instance of your web browser if you run windows.
2. If you run linux you're probably knowledgeable enough to do a kill or kill -9.
3. The design of the nimp.org link actually helps you to close all of the web browser windows that pop up. The default behavior of windows is to "group similar taskbar buttons" together. You may have noticed this from the old tabless-web browsing when you had many instances of (gasp) IE or another browser open. Windows would annoyingly consolidate all of the windows into a single button on the taskbar. To switch between windows, you needed to click on the button and a drop-down menu would appear. So the nice thing about this grouping is that after enough nimp.org browser windows show up, windows consolidates them, and you can simply right click on the one button that represents the dozens of web browser windows. Select 'close' from the drop down menu and you're good. In fact, I keep the "group similar taskbar buttons" feature turned on just for trolls like the AC who posts the NIMP links. That, and tabbed browsing lets me run only one instance of a web browser instead of running one instance for each open web page. If you want to change the behavior, right click on the taskbar and select properties, and you'll see a menu with the 'group similar taskbar buttons' checkbox.
Just wanted to bring some attention to this instead of sweeping it under the rug, and also wanted to provide advice on how to close your browser easily if it hits you.