Hi, thank you for the report. Our team is investigating it.

Reinstalling Firefox fixed this issue for some users.

The team wants to fix it with the next update.

A couple notes:

• Once the fix is released, you may still see an issue on your next immediate launch. However once you update your search engines again, the problem should no longer occur (IOW: you may need to fix your default engine one last time after the fix)
• In the meantime, users can also try deleting all their custom engines and re-adding them. This may resolve the issue for you as well without an update.