I got most of my coding ideas from the O'Reilly page I linked earlier and Netscape Developer, since I use Netscape almost exclusively at FR. Left, Right, Top, and Bottom Width tags are mentioned, so I gave them a try. But I'm thinking they are a buggy implementation.
I did try to move the order of the tags around, tried different values, etc, but I still didn't have any luck. So, even though having the top and bottom lines around a block quote looks fantastic to me, I think I may give up on it. Of course, this depends on what steveegg found.
Finally, steveegg, I did correct that "missing space" after the semi-colon in the border-style tag, but it didn't help.
Must be differences between your copy and mine that aren't reflected in the version number. Say, do certain threads (example thread here) not load completely for you? My copy of NS 4.77 stops loading after #55 with any CSS I have loaded, though it does load the entire thread without any CSS.