Undo Button Missing from Editor Toolbar

The Undo button disappeared from the formatting toolbar on my editor a few days ago. CTRL+ Z still seems to function.

 

Norton Forum Undo.jpg

 

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MS Windows 32-bit Vista Home Premium SP2 * Firefox 28.0 * IE 9.0 * NIS 2013 v. 20.4.0.40
HP Pavilion dv6835ca, Intel Core2Duo CPU T5550 @ 1.83 GHz, 3.0 GB RAM, NVIDIA GeForce 8400M GS