As I understand it, SNI slicing helps obscure the presence of the SNI field in the TLS ClientHello by splitting it into two chunks. The idea, it seems, is to make it more difficult for third-party network filters or DPI systems to detect or extract the SNI reliably.
I was thinking along the same lines. At this point, I have:
Removed all URLs in about:config
Disabled browser updates
Disabled all telemetry settings I could find
Despite all these actions, the browser still sends this request on startup. I'm starting to think it might be a setting that can't be changed via about:config.
Aux armes, citoyens,
Formez vos bataillons,
Marchez, marchez !
Qu'un sang impur
Abreuve nos sillons !