Here is what the script looks like in action:
You'll see a floating yellow box at the upper left corner of the browser window, telling you how many seconds remain until the page is reloaded.
Firefox with the Greasemonkey Add-On
Chrome with Tampermonkey
Go to here and click the install button.
By default the script will refresh http://football.fantasysports.yahoo.com/ every five minutes, but if you add any other URL to the include list, it should also work.
This script and others I've written are also on github.