How to do it: simply go to the Google Voice website (https://www.google.com/voice), then right click on the tab and click "Pin as App Tab." In Firefox, if a pinned tab changes its page title (to put a "(1)" in it for example), the tab will highlight as its attempt to notify you that something has changed.
So if you're a Google Voice user and you want to know when somebody texts you but you don't want to use Google Chrome, this method is easy and works pretty well.
Incidentally, if you pin Facebook, its tab will also highlight if you get new messages or notifications. It works on any web page that notifies you by changing its page title.