Upon starting it up, FireFox will by default reload all of the tabs that were open at the time that FireFox was last closed by using its built in Session Restore feature.
You can change this start up option in FireFox by clicking the word FireFox in the upper left corner followed by Options > Options > General tab and choosing from one of the three options provided in the Startup section.
- Show my home page
- Show a blank page
- Show my windows and tabs from last time
Choose either Home Page or Blank Page. Also:
- Make sure there's only one address in the Home Page
- Close all the tabs (except for the last one of course) before closing FireFox.
In more older version of FireFox, you'll find these same options under the Tools menu > Options > General tab.