Try running Firefox in [[Safe Mode]]. If it functions properly in that configuration, then one of your add-ons is the culprit. Incidentally, does the background (the purple color) look the same as the one in the screenshot when you access your Yahoo account? Yahoo has recently upgraded their mail program and it would appear that this is now the default color. If yours is a different color and you haven't modified it yourself, then the problem may possibly be related to the old version.