WordPress crop tool greyed out

The wordpress crop tool is greyed out for me in Firefox 3.6 (and IE7/8). I have disabled firefox add-ons and tried the solution here:

WordPress Image Editor not working – conflict?

But this hasn’t resolved the issue. Anyone know how wordpress determines whether to enable the crop button?

WP version is 3.0.1

thanks,

For me, the button became available once I clicked and dragged the mouse cursor over part of the image to select an area to crop.