Close all Chrome browser or Firefox browser instances at once

Under each browser, if you go to their respective menus, eg File menu for Firefox and Customize and Control Google Chrome (the wrench icon) for Chrome, click the Exit.

It will shutdown all instances.

This works for all browsers, if you replace chrome.exe with the appropriate image name:

taskkill.exe /IM chrome.exe /T

or if you want to force termination:

taskkill.exe /F /IM chrome.exe /T

See http://technet.microsoft.com/en-us/library/bb491009.aspx