One of the coolest and least documented features of Firefox is the ability to set multiple sites as your home page.

How you do this is, you open the pages as tabs of a single window, then in the General Pane of the Preferences, click on the Use Current Pages button.

Now these pages all open as multiple tabs of a single window whenever you open a new window in Firefox. This is particularly useful if there are certain sites you visit regularly – these sites can be set to be opened automatically when you launch Firefox or open a new window.