Hi, Firefox has had the ability to restore the open tabs at startup after a crash, and a Restore Previous Session item on the History menu, since the early days. Many years ago, a checkbox was added to the first section of the Settings page to do it automatically even if Firefox didn't crash. And in recent years, Firefox integrated with Windows and Linux features to restore your session if you shut down your system without first quitting Firefox. So I wonder why you never saw this before -- maybe your settings changed?
Startup Setting
You can use the first section of the Settings/Preferences page to restore or not restore the previous session windows/tabs during a normal startup. Please check this to make sure it reflects your preference.
Is Your Firefox Crashing?
If you are getting your previous tabs back even with that setting turned off, it's possible that Firefox is doing a crash recovery because it didn't shut down correctly. You may want to post that problem on the Mozilla Support site. They have a team of support volunteers to troubleshoot malfunctions with your currently installed version of Firefox, while this site is focused on idea submissions for future versions of Firefox (looking ahead anywhere from 8 weeks to many months). Here's a link to the new question form (for Windows/Mac/Linux): https://mzl.la/3y1vQUQ and the following article describes how to access crash reports:
https://support.mozilla.org/en-US/kb/firefox-crashes-asking-support
History Settings
What is your preference on saving browsing history? Do you want it to be saved between sessions? Firefox uses your browsing history setting to also control whether it save open tabs (session history). If you set Firefox to clear browsing history when it closes, that also will clear the session history files. But if you want to keep the URLs without restoring tabs, you may need some detailed preference tweaks. Best to ask on Mozilla Support about that.
