[mozvalentin]

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.

[krono]

Captive portal detection was only brought up by the non-constructive commenter and isn't part of the problem.

When the setting labelled "Allow Firefox to send technical and interaction data to Mozilla" is unchecked, the browser still sends out technical and interaction data through parameters added to various browser-provided/generated URLs (e.g. search results pages from built-in search providers, Firefox documentation links provided by the browser, DevTools compatibility panel links to MDN, etc.), by sending out pings when hovering over or clicking interactive browser-native interface elements, and in several other ways.

In summary: that setting is non-functional and should either be fixed, re-labelled, or removed entirely.