One possible reason for this is an instruction in a user.js file. Firefox checks for a user.js file at startup, and it will override your own preferences. This article describes how to check for this possible problem: How to fix preferences that won't save.

Another possible reason is if Firefox is not shutting down cleanly, and is always recovering from a perceived crash. To see whether this is the issue, you can change what Firefox does after a crash so that instead of automatically showing previous session tabs, it shows you a list of tabs. Here's how:

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

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

(C) Double-click the browser.sessionstore.max_resumed_crashes preference and enter 0 (that's a zero) and click OK.

The next time Firefox starts if you get the screen saying "this is embarrassing" and showing a list of your last session tabs, then the problem is that Firefox is crashing during shutdown.

Does any of that help with your startups?

You can check if you have an 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 this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)