What you can do is drag addresses from the Firefox address bar into a Finder folder, which will create .webloc files for each of them. Then open System Preferences > Accounts > [your account] > Login Items and drag the .webloc files into the list. Technically these will open on login, not bootup, but if you only use the one account it should work fine. Also, you'll probably need to have Firefox set as your default web browser.