I really appreciate having an open source solution. I hope the production costs can come down over time. I currently have an Airthings Wave and a Plus - which are roughtly in the same price range but additionally measure Radon.
This is entirely server-side UA sniffing going wrong. You get an empty HTML doc, only a doctype, with a Firefox Android UA. You can reproduce this with curl and it seems that this affects all UA strings with versions >= 65. <=64 work.
There is a distinct difference between telemetry and the captive portal requests Firefox makes. The captive portal checks have nothing to do with telemetry and contain no payload, which is why they are not covered by that setting.
You can actually see all of the telemetry data Firefox is sending by going to about:telemetry in your browser.
You can also see all of the probes it could be collecting here: https://probes.telemetry.mozilla.org/
Keep in mind that the telemetry that is captured in Firefox goes through a thorough review process, so that it respects user privacy: https://wiki.mozilla.org/Data_Collection