There are two ways you could do it, depending on whether you want to keep the original homepage setting or not.

The easy way, if you're not worried about losing your homepage:
1. In the Firefox menus, choose "Tools" then "Options", then "Main" (not sure what the icon is for it, I'm using a different skin than the default one, but it's the first choice at the top).
2. If Fast Dial is the only tab open, just click "Use Current Page" under "Startup". If not, enter "chrome://fastdial/content/fastdial.html" into the "Home Page:" box.
3. Hit OK.

That's the easy option, but you lose your homepage.

If you want to keep your homepage:
1. In Fast Dial, right-click an open area and choose "Preferences" to bring up the main prefs box.
2. Click "General" at the top (the wrench & screwdriver), then check the option "Show Fast Dial in new tabs" if it's not already checked. Then click Close.
3. In the Firefox menus, choose "Tools" then "Options", then "Main" (not sure what the icon is for it, I'm using a different skin than the default one, but it's the first choice at the top).
4. Under "Startup", next to "When Firefox starts: ", choose "Show a blank page" and hit OK.

This will open up Fast Dial in all new tabs, including the one you start up with. But this way, if you click the "Home" button on your toolbar, it still opens your original home page.

Hope this helps!