Joss has posted online in the past at The Bronze, The Bronze Beta, Buffistas.org, Fireflyfans.net, the official Delphi Firefly forums, and Whedonesque. If you want to see what he had to say at some of these places, check these out:
The Bronze VIP Posting Board Archives These archives store posts from several Buffy VIPS , including Joss, dating back to 1998. Posts from Usenet, The Bronze, The Bronze Beta, and The UPN Bronze can be found here.
Joss’s Fireflyfans.net profile This has a link to the only Ff.net post Joss ever made.
Buffista Archives If you’re feeling super ambitious, you might find posts from Joss here, most likely in the Firefly threads. Download them and do a Find in page… for “joss -” to find them. (If you just search for “joss,” you’ll find way more posts than is manageable.)
Whedonesque joss posts tag The folks at Whedonesque are very good about tagging any time Joss posts or comments, so it’s super easy to find links. If you’re on a thread and you want to find his comments, do a Find in page… for “joss |”.