Firefox should use the setting on the General panel of the Options dialog. This article describes those settings: Startup, home page, tabs, and download settings.

If you already have Firefox set to "Show my windows and tabs from last time", make sure you are not using automatic private browsing. In that mode, I don't think your session can be resumed.

Automatic private browsing is enabled if you set Firefox not to remember any history on the Privacy panel of the Options dialog.

orange Firefox button (or Tools menu) > Options > Privacy

To have finer control over privacy settings, you can choose "Use custom settings for history".

Any luck?

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

See:

• http://kb.mozillazine.org/Preferences_not_saved