When I look up my Google Voice number, it shows up as VoIP (not quite landline) with the carrier being Grand Central - SVR. It seems likely that Mozilla's service will similarly show up as VoIP. Some places are filtering VoIP numbers from their SMS verification schemes, but most places will let you sign up for promotional texts from a VoIP number.
There was something recently around a game which was able to detect if a user's phone number was on a prepaid or postpaid plan. I had no idea carriers share this information with others.
