I see that the version of dojo distributed with mango is ver 0.4.2. from March 2007. The latest release is version 1.2.3 and is a few days old.
I have trouble with the Tree in point hierarchy when I am selecting multiple points and dragging them into folders. My firefox CPU usage goes to 100% and it freezes for a little while. I know the the tree widget and drag'n'drop have had a re-write since 0.4.2, maybe around 0.9.
Have you thought about updating to a newer version of dojo? I know the latest version isn't backwards compatible to 0.4.2 so it probably wouldn't be straight forward.