This is one of the strangest things about Foxie, and we don't know exactly why she does it: is it because her hind-legs are disproportionately long (they are) or because her backside is very high (it is) or did she just pick it up through careful observation of us humans sitting on our chairs and sofas in the early days of her pupdom? My mother's unscientific view of things is that she was a mermaid in her last birth.
(Note: the photo above doesn't capture the best version of the pose, which is when she puts up just one of her hind legs on a chair seat and allows the other one to dangle. If I get a better picture at some point, I'll put it up here.)
Another possibility could be that Foxie greatly treasures her behind and wishes it to be in a state of maximum comfort always. (The first thing she does each morning is come to me for her daily back massage, turning her head sternly if the rubbing isn't vigorous enough.) But it's still not a very dog-like way of sitting.

"But how can he come forward so?" said Dorlas. "Lithe he may be, but he is a great dragon, and how shall he climb down the one cliff and up the other when part must again be climbing before the hinder is yet descended? And if he can so, what will it avail us to be in the wild water below?"And while I'm on the subject, a few more pictures:
Bow-zzat?

Afternoon siesta

In which we are very aware of the camera

Expression of bliss as evening head massage occurs

How many other doggies have such a big pile of books to bury their chewy bones in?