Open two home pages in Firefox

Under menu *Tools * → Options, go to the "Main" tab.

Under the Home Page text box where you key in your home page, just separate addresses with the pipe symbol (the one above your Enter key usually).

E.g., www.google.com | www.huffingtonpost.com | superusers.com

will open up three tabs whenever you start Firefox.

Alternatively, open all the pages you wish to load automatically when Firefox starts, and then go to menu Tools → Options → Main → Startup. Make sure to select When Firefox starts Show my home page and hit the button Use Current Pages.