Here it snaps back to Bing, nevertheless I had set Startpage as my default search service.

Thanks that's a helpful workaround. I'm switching to a different browser until they fix this bug, however.

Hi Sparky,

Our team is aware of this issue and has been working on a fix. The current accepted workaround is by reinstalling Firefox. Would you mind to do that and report back? Otherwise, you may need to wait for a fix that we're hoping to include in the 144.3 release.

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.

Reinstalling Firefox fixed it for me. Thanks!