Firefox 3.0.8, and IE 7.0.5730.11 both...
I don't think its particularly complicated. Maybe if I simplify it:
* Log into
http://beta.runbox.com
* Title (as in bar along the top of the window) says "RMM - Inbox (0/137) - Windows Internet Explorer"
* Click on Sent folder and it says "Loading" for a bit, then switches to Sent folder but title doesn't change
Separately, I just noticed my CPU (AMD 1500XP Athlon so no beast) maxed out for 3 or 4 seconds when changing folder - could explain the slow speed. I have around 40,000 messages between 10 folders.