Actually I think this might not be too hard. Cloudflare has a relatively small, published list of IP addresses: https://www.cloudflare.com/ips/ and Firefox has a DNS API for extensions: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web... (and kinda mostly not chrome, dev channel only: https://developer.chrome.com/docs/extensions/reference/api/d... )
That's not too hard to plug together, and Firefox even has stuff like `["offline"]` so you can avoid sending any requests at all.
So yea, that strategy seems to work. PoC functional, gonna clean it up later.
So yea, that strategy seems to work. PoC functional, gonna clean it up later.