Sorry, I didn't realise the logs were in moderation.

From the logs you posted, this is the critical part:

``` SearchSettings: get: No settings file exists, new profile? Object { operation: "open", path: "/Users/garycohen/Library/Application Support/Firefox/Profiles/5tcldzba.default-1492542418983/search.json.mozlz4", unixErrno: 2, stack: "", fileName: "(unknown module)", lineNumber: undefined } SearchSettings.jsm:115:18 ```

That's saying that the settings file doesn't exist. That would be enough to cause the default to be able to be loaded.

I'm assuming that you didn't remove the file yourself whilst restarting. Therefore could there be some application on your computer that is doing cleanup, or maybe removing the file?

Maybe a cache cleaner, or anti-virus?

The fact that the file wasn't found is my fault. I think I did remove the file before I copied the log files. I was confused as to the order in which to copy the file before using FF. My bad.

Do I need to make a copy of the file search.json.mozlz4 before I change the default search engine or after?

If you didn't remove the file, then something else did. So we need to figure out what's doing that. Hence my question about cache cleaners or anti-virus.

I suggest opening the profile folder, doing a normal Firefox exit, then checking to see that the search.json.mozlz4 file remains there until the next startup and isn't removed.

You're right! i run an app called cookie that automatically deletes cookies and cache whenever I close a browser. I disabled automatic removal and voila —my browser of choice was retained!

Thanks so much for your and everyone else's time and patience in helping me!

I'm glad we found it. Do you know the exact name of that app or where you got it from?

I'd like to see if we can follow up with them to prevent this from happening to other people.

It's called Cookie, by Sweet P productions. I just had to fine tune the settings for Firefox so that it didn't delete "other search engines." I'm running v 5 since I can't run v6, which is optimized for Big Sur, and I'm still on High Sierra