Firefox resets my search provider settings to the default after creating a new profile approx. every 6 months. It adds the removed providers Twitter and Ecosia and removes added providers like Wikipedia (de). Some search added providers remain (e.g. my personal dokuwiki instance).
The reset seems to happen between restarts of the browser for no apparent reason. I experienced this 5 to 10 times over the last years, i.e. in a few Ubuntu releases.
Since the issue happened today, I could make sure that it's not connected to add-on updates (bravo Firefox for no providing a table view of add-ons with easy accessible information
3 Reset to default
