You need to change your profile so that on opening you get a pre-chosen home page or a blank page.

The easiest way to do this is with the 'menu bar' visible Tools -> Options -> General That leads to a 'startup' dialogue including 'When firefox starts' Chose the option for a blank page or a home page, instead of previous tabs.

See Startup, home page, tabs, and download settings

cor-el (https://support.mozilla.org/en-US/user/287) told me about this:

"Tools > Options > General > Startup: Don’t load tabs until selected"

(It's a check-box)

Good point, if firefox 8 or later is in use
(Currently 3.6 and 9 are supported) and the op was using firefox but that was over a year ago.

See also

• http://www.mozilla.org/en-US/firefox/8.0.1/releasenotes/

including:

Added a preference to load tabs on demand, improving start-up time when windows are restored