By default, Firefox allows the setting of multiple homepages separated by the pipe or vertical bar character "|" (Note: it appears as a broken bar "¦" on many keyboards).
Now this works fine for opening multiple tabs every time the browser is started or the home button is clicked; however, it can slow down the browser startup. If your aim is to keep track of a few different pages throughout the day, but not necessarily all at once, then this extension is for you.
Homepage Randomizer is extremely lightweight and has a clean user interface. It adds a checkbox in the startup preferences section in Firefox (Tools -> Options -> Main). Simply add your multiple homepages as described above and enable the checkbox.