Here's a handy tip for those who regularly check multiple sites at the top of their day, or as soon as they hit their browsers.
Firefox parses URL's into multiple tabs when separated by the "|" character (shift-\). Hence, you can enter multiple url's as your homepage by entering:
https://www.google.com/ig | http://www.yahoo.com | http://lifehacker.com
If you use Autohotkey, you can also use it to open multiple tabs by the same syntax:
run, https://www.google.com/ig | http://www.yahoo.com | http://lifehacker.com
Of course, use discretion when you start up Firefox with multiple tabs; opening up too many at once could slow Firefox down significantly.