In your Windows Defender Firewall configuration are there any currently existing rules for Firefox?
Inbound, Outbound, Connection?
My Windows Defender has a couple of existing rules in all categories.
Likely a Connection rule along with some Inbound or Outbound rule.
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If you run "nslookup bing.com" (without quotes do you get the following results?
PS C:\Users\XXXX> nslookup bing.com
Your requirement is clear and straightforward and I am finding it interesting that there does not seem to be some immediate "How To" tutorial.
Here is a link that may prove helpful....
My current thought is a Powershell script that blocks bing.com if Firefox is running.
Still that all seems to be a bit cumbersome in some ways. Do you have any sort of test environment to work in?