Currently, if you delete the US english dictionary it gets reinstalled everytime you update firefox, and if you just deselected it gets reselected as default when you update. You should be able to easily remove US english dictionary without deleting files manual, and as long as another dictionary is install, it should get reinstalled every time you update (as they just discourage people from updating).
Firefox should also respect preferences between dictionaries across updates, restarts, different website etcs. You should not have to manually deselect US english and select another dictionary instead every time you type into a new website. There needs to be a way of changing the global default, permanently. The USA isn't the centre of the universe!