Same profile? It's not recommended to use upgraded profiles in older versions. In fact, it's normally prevented by Firefox.

Possibly Firefox 76 changes the file in a way that makes it unusable in Firefox 75. I haven't compared. If you want to look at the JSON inside the respective files, you can decompress them on:

https://www.jeffersonscher.com/ffu/searchjson.html

Come to think of it, if the format is changing, the function of the page to make a list might not work any more.

For quite some time now, I've been using the shortcut target command

"My Documents\FIREFOX\firefox.exe" -p --allow-downgrade

to avoid the complication of switching back and forth. I've never had an issue until now. Going "backwards" causes it.

Possibly Firefox 76 changes the file in a way that makes it unusable in Firefox 75

Also, every time it happens, the same four, random search engines remain.

Is the other profile a Firefox 76 or 75 (or older) profile ?

Is the other profile a Firefox 76 or 75 (or older) profile ?

I hope this makes sense.

It was pre 75, but if I transfer it to 75, and launch 75, where it is fine, then copy and save the file in that profile elsewhere, then open the profile with 76, then revert back to 75, that's when the corruption occurs. I then close 75, and restore the file I saved from 75, and it is normal.

The transition from 76 to 75 breaks it. I put back the corrupted file, then open 76, where it continues to show only 4 search engines. I close 76, replace the file with the one I saved from 75, launch 76 again, and it work properly in 76.