I use a Pi Hole as a DNS sinkhole and have added a couple of internal domains to access some servers. This works on other browsers but not on Firefox. I did find the DNS over HTTPS setting and turned it off in the GUI, and also tried the configuration editor. But it still seems to use another DNS server. To confirm I blacklisted facebook.com and tried opening it in firefox and it worked :(. (I tested in 2 other browsers and the blacklist worked and didn't load the page). I found some reddit threads with a similar experience. I was hoping to switch to Mozilla but this is an impediment to me doing so.