@Derek: just wanted to say thanks for the hint. Actually, the JSAPI says that the argument to esri.hide/show is "The name of the HTML element", rather than the DOM element itself. Curiosly, passing just the element name (without enclosing in quotes) in Chrome and IE (!) works fine, but its Firefox that throws an error.
I couldnt figure it out, but from your note I now use dojo.byId to pass the parameter and it works in all browsers.
Rich.