Comment 1 for bug 310418

This may not be a firefox bug, but rather something to do with the KDE/GTK crossover - I'm getting similar problems with inkscape when trying to move objects with the mouse.

Basically, if the mouse moves, then inkscape seems to register that it's moved, but once it's stopped moving, it goes back to the starting position. If the mouse moves smoothly, it the object flickers between the original position, and the new position.