The Ajax client doesn't allow you to use that reference. However, that will work with the Plugin client. Note that the Plugin client is basically the same as the Desktop software, and uses the Windows OCX and ActiveX renderization, that is not available in the Ajax client, and hence some functionality doesn't work in this client (for example, some macros that draw objects, create groups, modify captions in charts and so).
There's not another client for different browsers than IE for the time being, and I don't think there will be, since QlikView is enhancing its Ajax client each version, instead of having thousands of clients/extensions for each possible browser version. Firefox cannot interpret ActiveX (native, I don't know if there is some extension that does), nor any other browser can.