In an ideal world yes, this should be implemented at the OS level. However, the vast majority of users would not see any benefit until OSX/Windows/Android/iOS/<

In an ideal world yes, this should be implemented at the OS level. However, the vast majority of users would not see any benefit until OSX/Windows/Android/iOS/<insert Linux flavor> receives an update for DoH-by-default and then users either have auto-updates turned on or manually apply the update. Until that time comes, this is a fine step in the right direction, and you can disable Firefox's DNS if you don't want it.