Actually, having just turned off the Menu Bar, it is still there. Still want to know what it is.

I Have it too. IT seemed to appear after i downloaded an add on called 'FoxTab'

Thanks for that reply... I already had FoxTab installed before upgrading to FF4 so it looked as if it was a FF4 thing.

On fiddling, I discovered that if you open the main FoxTab screen the little box vanishes and doesn't return when you close it.

I've had that for a while, too. I had FoxTab when I upgraded to Fx4, so I assumed it was Fx4. If you disable FoxTab, the square disappears (for me at least), so it must be a bug in that extension. If there was a function to enable/disable extensions, maybe with a list of checkboxes, that would make troubleshooting that stuff easier and quicker.