Losing open tabs in Safari

Go step by step and test.

1. If Safari is slow, stops responding, quits unexpectedly, or has other issues

https://support.apple.com/en-us/HT203353

Section: Manage Extensions

https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac

2. Delete com.apple.Safari.plist file and test.

Empty the Trash.

Quit all applications.

Hold the option key down and click the "Go" menu in the Finder menu bar.

Select "Library" from the drop down.

Library > Preferences > com.apple.Safari.plist

Right click on it and select "Move it to Trash".

Restart and relaunch Safari holding the shift key down.

If this doesn’t help, “Put Back” the com.apple.Safari.plist

Right click on the Trash icon in the Dock and select “Open”.

Right click on the com.apple.Safari.plist and select “Put Back”.

Select ”Replace” from the dialog when it appears.

Check Safari/Preferences/General to make sure Safari opens with is set to All Windows from last session. If that option isn't available, quit Safari, then try going to System Preferences/General and uncheck Close windows when quitting an app. Open Safari, go to Safari/Preferences/General and see if you can now set Safari opens with.