Firefox uses an encrypted DNS channel. Go to Settings > Network Settings at the bottom of that dialog you will see "Enable DNS over HTTPS" and under that the DNS provider it will use, which is Cloudflare by default.
For testing purposes: A. Try changing the provider to someone else that you know will work. You can use the "Custom" option to set it to whatever DNS you like. B. Uncheck the "Enable DNS over HTTPS" which will make the browser work like any other and use your network's default DNS.
See if one of those two things fixes your problem.
Thanks, I just checked and "Enable DNS over HTTPS" is unchecked.
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