JavaScript Error, Can't Send or Create Invoices

When I try to click the large "Send Invoice" button in the top right corner, and also when I click the link below that for "Send Invoice," I get a JavaScript error. I am therefore unable to send and create invoices.
(Actually, I can CREATE an invoice when I go to the client list and click the tiny "create invoice" text link.)

I am using FireFox 3.0.13.