Since a few days, feeds registered in Thunderbird that are hosted on feedburner are not retrieved properly anymore. When looking at the thunderbird devtools, I see that a 403 response is returned.
Reproducing the same request with curl, I also got a 403 response. I then managed to get the actual content when removing the "Sec-Fetch-Site: cross-site" header that is part of the Thunderbird request.
It would be great to unblock such requests so that feedburner feeds work in Thunderbird.