1. From the Firefox Tools menu, choose Options
2. Click on the text box under Homepage - Location(s) group (see screenshot)
3. Type the address of the webpage that you want to load when Firefox loads. Now insert a vertical bar (pipe) | (generally located beside the backspace key on Qwerty keyboards). Next, type the web address of another webpage that you want to load. You can insert any number of pages separated by a pipe symbol.
The next time you start Firefox, each of the webpages will open in separate tabs.
Another Quick alternative for launching multiple webpage on Firefox startup
1. Open Firefox browser and load all favorite webpages in separate tabs.
2. Choose Tools - Options - Homepage - Locations. Now click "Use Current Pages"
Firefox will automatically make all the currently open tabs your home pages.