1. TRR is not turned on by default. To turn it on, you need to go to about:config and set network.trr.mode to something other than 0 or 5.
2. Even if trr.mode is turned on, you need to go in and set the DOH server at network.trr.uri. The default is blank. You can set it to any publicly known DOH server (https://github.com/curl/curl/wiki/DNS-over-HTTPS#publicly-av...), or even your own.
3. The article doesn't talk about how your ISP can use DNS to censor your result - very common, for example, in a country like India where the court orders certain sites taken down. Mozilla's DOH solves this.