Why is UNDO CLOSED TAB disabled by default?
Whenever my browser is updated, the UNDO CLOSED TAB is disabled by default. At that time, I need to go to about:config to enable them.
Please see the article How to fix preferences that won't save.
You may have a user.js file setting that preference each time you start up Firefox. Information on how to remove it is in the article above, or more specifically this section:
If you have no user'js file, the article shows other possible reasons why the preference keeps getting reset.