In this post, i will talk about configuring firefox browser to use DoH and not to fallback to OS native resolver
Step 1 :-
Configure DoH like the way it is mentioned here - https://support.mozilla.org/en-US/kb/firefox-dns-over-https .
I am using https://doh.nkn.in/dns-query (which is the DoH server of https://www.nkn.in and in alpha testing stage) as my DoH server in firefox browser.
Step 2 :-
Check you configuration by typing about:networking#dns in the browser address bar. Most probably, you will see something like this --
Here, the DoH Mode option handles how firefox manages your DNS queries. This document (https://wiki.mozilla.org/Trusted_Recursive_Resolver) talks about the options available under 'DoH Mode'.
So, if you want to use only DoH and not to fallback to native OS resolver, you can change the value of this parameter to '3'.
Step 3 :-
For changing the value of 'DoH Mode' parameter, type about:config in the browser's address bar and search network.trr.mode and click on edit icon and change the value to '3'.
Now all the DNS Queries from the firefox browser will go to DoH server instead of native OS DNS. Make sure that DoH is available and accessible to you otherwise, you will not be able to access any site as we have disable the fallback to native OS functionality by changing the DoH Mode value from 2 to 3.