It is possible to change the url, however I'd go the route of changing params. Don't forget you can also you baseParams if you have info you want to send with all XHRs. Should you choose to change the url see the Grid FAQ on how to do it.
Don't specify the same baseParams that you are specifying in your load params. baseParams are parameters that will be passed with the same value on every request. The params you specify in your load are params whose values are expected to change every time you do a load().
One other (harmless) error is that Ext.data.Store doesn't have a method property in its config. I presume that this is because it always needs to POST parameters so there is no sense in doing anything else.
Is Firebug acknowledging that an AJAX request was made or are you failing before that can even happen? If you are failing prior to request time what is the error console telling you?