You can check for problems with the sessionstore.js file in the Firefox profile folder that store session data.

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

Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost and you will have to recreate them (make a note or bookmark them if possible).

Hello,

In order to solve your problem please follow these steps:

1. Click on Open Menu
2. Then click on Options
3. Next click on General
4. Select the type of page you want When Firefox starts

Report us back if this work.

Sometimes third party software creates an optional settings file named user.js and that file can contain an override which forces Firefox to restore your previous session instead of following your setting in the Options/Preferences dialog. This article describes how to investigate whether you have a user.js file so you can remove it: How to fix preferences that won't save.

By the way, is this problem on a system running Windows, Mac, or Linux?