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.
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.
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.