Investigate new Search Engine configuration
While working on #33533 (moved) I noticed that the engines configured in
list.json were being ignored, and had to set the
browser.search.modernConfig = false for them to work.
I believe this is because of a new Search Engine configuration that has been enabled by default in nightly recently, this is the meta ticket is https://bugzilla.mozilla.org/show_bug.cgi?id=1542235.
We should investigate this and see whether it's enough to flip that pref to get the behaviour we want.