We're running into a problem that appears exist in the current ExtGWT 2.2.6 code base and we're looking for a work around. The issue only exist in FF (it works as expected in IE).
Currently we're using Viewports to load our app. The problem first surfaced when we had a HTMLContainer in a BoarderLayout that needed to scroll because it had a lot of text in it (think of an e-mail client with a grid on one side and a reading/preview area on the other). In the HTMLContainer (in FF only) we are able to click on it to make sure it has the focus and then press the down arrow and the whole page shifts up by 10-15 pixels. Upon finding this we tried it elsewhere throughout other Viewports we render and found the same issue exist there as well.