Hmm, could you check this setting:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste startup and pause while the list is filtered

(3) Double-click the browser.startup.page preference to display a dialog where you can check/correct the value:

• 0 => blank page
• 1 => your home page(s) {default}
• 2 => the last page viewed in Firefox
• 3 => previous session windows and tabs

If yours has changed to 2 or 3, change it back to 1 and then click OK to save the change.

Firefox also might be doing an automatic crash recovery. To spot that issue, you can change what Firefox does after a crash: instead of restoring open windows/tab, it can show a page with a restore button. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste sess and pause while the list is filtered

(3) Double-click the browser.sessionstore.max_resumed_crashes preference to display a dialog, change the value from 1 to 0 and then click OK

You can check if you have a user.js file in the Firefox profile folder that sets the browser.sessionstore.resume_session_once pref to true.

• http://kb.mozillazine.org/browser.sessionstore.resume_session_once

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox