That's is a Addon for Firefox browser if there is issue you need to contact the Addon developer for help otherwise you need to go to Firefox extension manager and remove that Addon.

I DON'T KNOW IF I CHOSE CORRECT TOPIC, BUT I CHOSE THE DARK READER MODE THINKING IT MIGHT BE USEFUL FOR MY EYES. BUT I DON'T REALLY CARE FOR THE DARKNESS OF IT, AND I CERTAINLY DON'T WANT TO HAVE TO PAY FOR IT. BUT I'M NOT SEEING WHERE I CAN DISABLE THAT AND GO BACK TO THE WAY IT WAS BEFORE. PLEASE ADVISE.

https://support.mozilla.org/en-US/kb/firefox-reader-view-clutter-free-web-pages

Click on the Aa on left and select light, dark, sepia or auto.

That's is a Addon for Firefox browser if there is issue you need to contact the Addon developer for help otherwise you need to go to Firefox extension manager and remove that Addon.

This has nothing to do with an extension as it is about the Reader View in Firefox as you can select light, dark, sepia or auto. See the knowledge base article link in my previous reply.