It works Rated 4 out of 5 stars
I'm running Firefox 9.0.1 Mac. I tried this despite recent comments claiming that it doesn't work, and it works perfectly, aside from the location bar having focus (unlike when you open a new window). No idea why anyone would think this option shouldn't be built-in.
Edit: "My Homepage" puts the focus in the page rather than the address bar, as mentioned by other users. Use that instead, if this is a problem for you.