Did you change it here specifically ? about:settings#search

Which search engine do you use ? Maybe some nasty one keeps setting itself back as default ?

I keep having to go into setting and changing the default search engine to my preferred one ‘Ecosia’. It works for a couple of sessions and then changes to ‘Bing’ or another without me touching the settings

This question is about Firefox for iPhone, but you're asking in "Firefox for Desktop" section.

The team will fix this bug with the next update (version 145).

A couple notes:

• Some users say that reinstalling Firefox fixed this issue for them
• 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
• 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.