Tab Effect is a novelty extension, but seems to be very popular (at least at my office). Using DirectX 8 adds an effect when the current tab is changed. The effect is hard to describe, but there is sample picture on the AMO page. Not sure how long I will keep this one installed before the ‘novelty’ wears off. Tab Effect works with Firefox 1.5-2.0.0.x and is about 38K.