Earlier quoted context omitted.
I think the burden here is backwards? URLs may contain Protected Health and other Identifying Information. If this data leaks SSL and could be sent to a 3rd party, then it makes Firefox an unsuitable client for a great many applications. EDIT: OK. It's boolean flags (like use of flash) plus an eTLD+1 (example.org; not myname.example.org?). Even so, I believe this tracking should be opt-in with a disclosure screen tha…
They don't plan on collecting URLs, just (eTLD+1). The only real issue I can see here are users who have registered their own domain under an eTLD, and have it set to their home page. eTLD: https://en.wikipedia.org/wiki/Public_Suffix_List
Doesn't the differential privacy system described above prevent even that from being an issue?