I'm facing the same problem. I also could not reproduce the error on chrome or safari, only on firefox. (I will check IE later).
I have two tabpanels, each contains one chart and one grid and a global update button. By pressing the update button one store would be triggered to reload. Within the load event of the store the json data would be distributed to the other stores (which are connected with the grid/chart components).
I will try to detach this part from the whole application in order to provide an example.
This happens to me in Firefox when I create a chart with the "renderTo" property pointing to a div that has the "display: none" attribute. Works fine in Chrome, IE and Opera.
I suspect it has to do with FF not providing size information to the chart rendering code when the div is hidden, but I haven't investigated too deep. A workaround is sending the containing div out of the viewport using the venerable "position: absolute; left: -9999px" trickery.