Firefox provides options to the user to retain the last accessed web pages when a Firefox starts. This can be done using the Startup section available as part of the Firefox General Options. To make this change, click the Tools menu and select Options from menu list.
Navigate to Startup section under the General tab and click the dropdown arrow next to When Firefox starts
This would display the following option values.
In order retain the last accessed web pages, select Show my windows and tabs from last time and click click OK button to confirm and save the changes.