If I go to http://music.kimiko-piano.com/album/bach-well-tempered-clavier-book-1 using Iceweasel, and then click the play button, the panel vanishes. I can bring it back up from the terminal, but if I press play again, it vanishes again.
Created attachment 6518 Errordump from xfce4-panel
Added an error dump from one time when this happened.
Maybe you use a weird font where some particular character is not supported? (That's what the error message suggests...) In any case, works for me with that particular page (and Firefox).