There is a difference between legitimate differences in behavior between browsers (which GWT will try to shield you from) and outright bugs in one specific browser (that should be fixed in that browser). What you described sounded like an actual bug in FireFox, so it should be addressed at that level. IOW, we don't feel that it is GWT's mission to fix every bug in every browser.
OTOH, if you can show that the behavior you are seeing is caused by an actual bug in GWT itself (maybe bad JS generation under FF in this case), it would be something we would want to address at the GWT level.