If you’ve set  Firefox to show windows and tabs from last time under “General” options for Startup Firefox 8  allows you to choose the tabs to load at startup which helps in faster-startup times of Firefox, you can enable this option under options on the General tab, here is how.

don't load tabs until selected in Firefox 8 startup

From Firefox Menu>Options >Options>select General tab make sure “Show my windows and tabs from last time “selected for Startup and select don’t load tabs until selected option, click OK to save the changes.

P.S. This feature not working for me, let me know if it worked for you.