There has been some discussion for a time about how SSA (Server Side Appearance bakeing) has messed up avatar-offset. This is the offset most third party viewers provide that allows you to adjust the distance from your avatar to the ground or a chair when sitting.
Henri Beauchamp and a few others have been asking the Lindens to do something with the problem. It affects lots of pose balls and sit adjusted items in SL. As best I can tell the Lindens see this as a minor problem. So, there has not been a fix, at least not one that works for most people. The worst problems are being fixed, but…
The Firestorm Dev Team has removed the avatar-offset feature from the new 4.4.0 viewer just released. (At least I am told so in FS Support and I can’t find it) Watching the support channel this morning the top two question have to do with ATI users getting black screens (fix) and people asking where the avatar-offset feature was moved to… the answer: OUT.