Firefox/Thunderbird can't save temp files - how do I change their location?

I just used Migration Assistant to move an account from my old MacBook to a new MacBook. In the process, I changed the name of the account, which ended up being a bad move. After the move, I cannot download files/attachments from Firefox or Thunderbird - I get an error message stating that the temp file (always ending in ".temp") can't be created, because I don't have access to that folder. Well, the folder it's trying to access is the Desktop in the old account.

It doesn't appear to be an issue with those programs themselves, because even if I try to change the Download folder to an existing folder within the new account, Leopard still tries to create the temp file in the old account. I went so far as to create an account with that name, and give my new main account read/write access to that folder, and sure enough, it works. However, that's an awful workaround, as now all those temp files are accumulating in that account.

Bottom line: How do I tell Leopard specifically which folder to create temp files in?