translucent rectangle in top left corner when opening firefox

when first opening Firefox a small translucent rectangle approx 3cm x2cm appears in the top left corner it then immediately disapears

could this be a tracker?

I have googled to no effect disabling Addons then restarting doesn't work, and i cant find out how to disable hardware accelerator. I have completely reinstalled the os I have three on separate partitions I'm using ubuntu11.10 Atm but this happens on all my linux os' I haven't checked windows yet though any advise would be welcome.

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