1.85:1 is still "widescreen." "Full screen" is an outdated term that refers to the aspect ratio 1.33:1 (also referred to as 4:3), which was the standard ratio for television broadcast for decades. Once TV began broadcasting in HD, 1.77:1 (a.k.a. 16:9) became the standard, and it's what pretty much every HDTV is designed to show optimally.
A lot of movies are filmed in 1.85:1. I've heard that Joss went with it because it's better for the 3D effects (I don't know if that's true or not), and because it's better for vertical action (of which there is apparently quite a bit). It has nothing to do with Joss' "TV sensibilities." He filmed Serenity in 2.35:1, which is close to the usual cinematic aspect ratio of 2.39:1. 2.39:1 is most often the ratio people think of when they say "widescreen."