I have been using this workaround but I haven't seen use-application-dns.net .
in the logs any time recently.
I just tried it out on Firefox and the logs show:
mozilla.cloudflare-dns.com IP (1.1ms)
So firefox is not using the "canary feature"
I see this note on their website:
Note: The canary domain only applies to users who have DoH enabled as the default option. It does not apply for users who have made the choice to turn on DoH by themselves.
You should inquire with Mozilla/Firefox whether replying 'Okay' to that Firefox popup would qualify as making "the choice to turn on DoH by themselves.".
If they do so even if you choose 'Disable', this would warrant to open a bug in Firefox.