Thanks... I ended up getting it to work reliably with the two windows, here's how:
Start new firefox, load localhost then pandora... it'll rip for quite a while. The localhost tab changes names etc as the pandora tab changes songs. From what I understand of how pandora works this isn't particularly odd, as it's just reading whatever temp files it can.
When pj stops working eventually, firefox must be closed down and reopened, then it'll work for a while again. PJ might have to be restarted first if it's a socket error.
The only problem with this is reload doesn't work, since it refreshes the PJ tab and not the tab with the working player. I did try clearing the cache & cookies, same result, and didn't try editing the file as from the sounds of it that'd just remove the player that's there (doing nothing). Hopefully sometime it'll 'just work'.
As an aside, I couldn't get PJ to show up _at all_ in Minefield (nightly build of firefox). It threw some 404 type error. I had this problem a while back and am now using Firefox 2.0.0.1.