In bug 658829 jimm was easily able to animate the HTML undetermined progress meter because in that mode the progress chunk is sized to fill the meter. He wanted to apply his animation to the default themed XUL progressmeter as well but XUL layout doesn't special-case undetermined meters. I suppose the choice is between creating an extra XBL binding that just contains a full progress chunk or altering XUL layout so that the normal binding will create a full chunk. Either way the widget code can then animate it as per the HTML meter.
Created attachment 615043 [details] [diff] [review] Proposed patch
Pushed changeset 450d8cd16316 to mozilla-central.