Android does same-provider auto-upgrade if it determines that the recursive supports DoH (last I checked, if it's on Google's list). However, this means that unless you configure your own resolver, you're vulnerable to whoever controls the network substituting their own resolver. Firefox uses a set of vetted and pre-specified resolvers ("trusted recursive resolvers"), so is less vulnerable to this form of attack. I say "less vulnerable" because by default it will fall back to the system DNS on failure, but you can configure hard-fail.

You may or may not think this is a better design (I was one of the people responsible for Firefox doing things this way, so I do), but hopefully this explains the difference.

See: https://educatedguesswork.org/posts/dns-security-dox/ for more on the difference.

Yes, this. AND while Firefox is providing you the control to choose when to enable or disable DoH, you don't get that control at OS-level, or even the visibility of what the OS is choosing on your behalf for each such query.