Generally we have observed this to be caused by third party software modifying settings from outside of Firefox. In Firefox 145 released yesterday, we have added some detection that might be able to indicate that this has occurred. If you can update to 145, then the next time the search engine is reset, go to Help -> More Troubleshooting Information, under "Important Modified Preferences" look for `browser.search.lastEngineIgnored`. If it is there, then please copy and paste its value here - this will hopefully help us understand what's happening. Though it does indicate that it has been changed from outside.