Does the HTML Editor support border images in CSS3? Haven't been able to get it to work, but not sure if it's a Editor problem or a Firefox problem.

Border images in CSS3, I mean commands like the following:

border-image: url(border.png) 27 27 27 27 round round; or

border-image: url(border.png) 27 27 27 27 stretch stretch;

It uses IE's Trident rendering engine for previewing, so no. As for Firefox, like other browsers, its implementation of the as yet unfinalized CSS3 features picks and chooses which parts to actually bother with at this time. Have you seen something indicating that FF should be supporting this new border styling?