When I am viewing a message and look at the message tree under the message.
The distance between the highlighted message and the message directly underneath is reduced, compared to the distance between the other messages in the tree.
The reduction in space, between the highlighted message and the one below, makes the subject line of the message (below) difficult to read.
This only occurs with Firefox, IE does not have this problem

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic