Are you noticing any other settings being reset to their defaults?

Your preferred default search provider is stored in the prefs.js file, a settings file in your Firefox profile folder. Some "cleaning" software may remove this file at intervals, and some software that protects you from unwanted browser changes may replace it with an earlier copy of the file (e.g., Advanced SystemCare's Surfing Protect feature can do it). This article has more suggestions for diagnosing settings resets: How to fix preferences that won't save.

Thank you for your help!

I had accidentally solve the problem but I didn't really know how. You are right, I own the paid version of Advanced System Care. I noticed that a message came up asking me if I wanted to allow the recent changes the Firefox. I said yes. After that, the problem was gone.

It is really a big help to know about the prefs.js file. If It happens, I can go straight to this file and erase it. I wasn't paying attention to the other settings. I am not sure if they were put back to their defaults.

I'm on the fence as to whether to use Advanced System Care in the future. I really haven't seen a tremendous benefit but there seems to be a little bit of a downside.

Thanks again for all your help.