I'm on a Mac and don't recall Windows commands, but it should be similar.
On the Firefox menu bar, go to File --> Preferences --> General Tab
Look at the options for Startup, then choose one of the following: Show my home page, Show a blank page or Show my windows and tabs from last time.
* It sounds like you want to use Show my home page, but maybe something has changed.
Under the Home Page box, you'll need to choose one of the following: Use Current Pages, Use Bookmarks, Restore to Default
* Open only the tabs you want to automatically open, then select the Use Current Pages
I hope this helps.