In Firefox 3.0.x, the following test results in the bbar being chopped off, seemingly pushed off the screen by the canvas element. The bar is sized correctly if left as a tbar.

Code:
Ext.onReady(function() {
    var layout = new Ext.Viewport({
        layout: 'border',
        items: [{
            region: 'center',
            bodyCfg: {
                tag: 'canvas'
            },
            bbar: {
                items: [{
                    text: 'Item'
                }]
            }
        }]
    });
});