Recently, Thunderbird has started being "artsy-craftsy" when I switch between messages or folders, overly animating the change between the previous and the newly-selected item. It makes it appear initially that I have clicked on the wrong item! I don't know if this delay is a design feature, or is the result of a slowness in selection programming. Either way, it is annoying.
A clarification: When the previously-selected item has already been read, it is shown in a grey background. Before Thunderbird changes its focus from that entity, it momentarily changes its color back to bright blue (for no logical reason) before moving the focus to the new entity that I have selected.