Hello scally718! I am assuming you are talking about the "Restore previous session" feature. You can still find this in the menu on the top right -> Restore previous session! The automatic window only opens up when Firefox encountered some kind of error like a crash! You can find more about this over here! Hope this helps, and if it does, please mark this post as solved!

In case you use "Clear history when Firefox closes" or otherwise clear history.

• do not clear the Browsing History

• Options/Preferences -> Privacy & Security -> Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data
• clearing "Cookies" will remove all selected cookies including cookies with an "Allow" exception you may want to keep