On a lark, I decided to try and copy the font settings from Flock into Firefox and see if that was the underlying cause of my text-racing-ahead-of-cursor issue in OS X FF. Turns out that... it was. Amazingly enough, using the default Firefox/Flock font settings gets rid of the issue. So, don't set your default font to Verdana size=14 in FF on OS X or you'll have a tough time editing textareas.