I'm not privy to the search deal, but my understanding is that this is driven by our internal user research and metrics, which show that a significant number of users have a search engine other than what they prefer or expect. In these cases, users usually choose to directly navigate to their preferred search engine to search, instead of using Firefox access points. That’s a poor user experience, so our focus here is on ensuring users have the defaults they want.

The decision tree is roughly:

1. If the custom engine is one of our default options, keep it.

2. If the custom engine was set by an add-on, keep it.

3. If the custom engine uses HTTPS, prompt the user to actively choose by opening about:searchreset, and do not prompt again after the user has made a choice.

4. If the custom engine uses insecure HTTP, silently reset to the default.

You can open about:searchreset yourself to see what the prompt looks like.

Code at https://dxr.mozilla.org/mozilla-central/rev/a928be5dacc3b544...