This tends to happen when there’s some kind of DNS configuration issue, most likely DNS over HTTPS. If that’s explicitly disabled in Firefox and the problem still occurs, then there are some troubleshooting steps to consider:
Is Firefox up to date?
Is the system up to date?
Have you tried reinstalling the extension?
Have you tried a clean Firefox profile with only the extension installed?
Has the system been restarted since the problem began occurring?
This tends to happen when there’s some kind of DNS configuration issue, most likely DNS over HTTPS. If that’s explicitly disabled in Firefox and the problem still occurs, then there are some troubleshooting steps to consider: