I just checked, there is no simple DNS settings in firefox...
I have no idea what you were doing, maybe you tried to set up a Proxy?
There is DNS over HTTPS setting under Privacy & Security, where you can set up Cloudflare, NextDNS or Custom DNS. This works for me, but if you set this to "Max Protection", a lot of sites will not work, which is not firefox's fault.
You really should set the primary DNS in your router's settings, but if that's not possible (sometimes it's not an option), then the second best is to set it in your device's network settings.
I used to be attached to my username, it was unique enough to be used everywhere. I used it in the mindset of creating an online presence, so my friends recognize me in other games and such, but then slowly realized it makes you very DOXable and vulnerable in general.
Nowadays I use one username per hobby, except gaming. I want you to be able to find other stuff I created in that specific space, but I don't want you to find out everything about me.
EDIT: Also, my username was too unique, if you googled it, you would get hundreds of results, and almost all of them were me... Random accounts on sites, forum posts, etc.