PS. I added Marginalia as a search option (even the default for now) in Firefox Nightly (on Android).
In case others want to, under settings for search, you can add other, then name, and then:
https://search.marginalia.nu/search?query=%s
I did not expect this to not be available.
I know. It's an idiotic UX regression over being able to configure search engines yourself in the Settings screen. Takes control away from the user.