Lately I've found FireFox holding it for extremely brief periods of time until the mouse moves a little, at which time it flashes on and off again. When it's doing this, it only stays for a fraction of a second. I suspect this is what the OP is referring to. It's annoying all right.

Edit: I just checked and it's behavin what I would call normally here at work right now. The little box pops up for about five seconds, which is long enough to read what's in it. I suspect the problem I'm seeing with the very brief flash of the box is happening at home, where I have a newer version of FireFox.

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