Try this workaround I just posted to another site where someone had the same problem:
https://getsatisfaction.com/mozilla_messaging/topics/group_by_sorted_tree_collapses_for_unified_folder_when_switching_to_single_account_and_back
Try this workaround I just posted to another site where someone had the same problem:
https:/ /getsatisfactio n.com/mozilla_ messaging/ topics/ group_by_ sorted_ tree_collapses_ for_unified_ folder_ when_switching_ to_single_ account_ and_back