In this case please try to reload/refresh the page where you have this massage on.
Probably Midnight Lizard or Firefox were updated and caused the problem.
If it does not help: Are those pages with the message turned to dark?
If not: Are they represented by a regular website? (like Wikipadia and not like addons.mozilla.org)
Extensions does not work on addons.mozilla.org and all Firefox system pages.

There are keyboard shortcuts for toggling Midnight Lizard extension:
  ⮚ on current website: Alt+Shift+L
  ⮚ globally: Alt+Shift+M
And Firefox team is working on the UI to allow users to override the default shortcuts.

If you see this message on some heavy websites you can switch Midnight Lizard to the Simplified mode which works much faster. Also it is possible to turn it off for that particular website.
I'm making it better every version and in the next one I have some optimizations as well.