Unfortunately the effective tld service: 1) doesn't like you passing it no-dot hosts (say, "http://mochitest:8888/") 2) doesn't like you passing it IP addresses (say, "http://127.0.0.1:8888/") 3) is generally pretty picky. The code at https://dxr.mozilla.org/mozilla-central/rev/f4f374622111022d41dd8d5eb9220624135c534a/browser/extensions/webcompat/content/lib/ua_overrider.jsm#109 should be catching those exceptions and return early / ignore those cases, rather than letting those exceptions propagate, spam the console etc.

Thanks for the report. That is indeed an issue we have not thought about.

We generally don't plan on overriding local ressources and only target high-profile issues, which usually have a domain, so simply ignoring cases where we can't get the base domain seems like a good way forward. Eric, could you have a look please?

Merged into the GitHub repo, will be synced to m-c soon. Thanks again for the report.