From MozillaZine Knowledge Base
This preference controls the speed of "Downloads Complete"-style alerts that appear in the bottom-right. Specifically, it controls the number of milliseconds between sliding movements for situations were the alert moves and the number of milliseconds between fading changes when the alert fades in and out.
Possible values and their effects
Any positive integer. Thunderbird default: 50. All others default: 10.
First checked in
Has an effect in