Following steps will be helpful to prevent firefox from restoring previous sessions:
- In the Location bar, type about:config and press Enter.
- The about:config “This might void your warranty!” warning page may appear. Click I’ll be careful, I promise! to continue to the about:config page.
- In the Search box at the top, type browser.sessionstore.resume_from_crash.
- In the resulting grid, double-click on browser.sessionstore.resume_from_crash to set it to false.