The main feature of an MMO mouse is the number of programable buttons. The extra buttons can be assigned for custom tasks such as accessing inventories, consuming material, and issuing commands among many others depending on the game. The distinguishing feature of a dedicated FPS mouse is the high DPI or dots per inch. Basically, the higher the DPI, the more the sensitivity. So your cursor travels more pixels in a rather small span of physical movement of the mouse. These days, some FPS mice feature upwards of 12,000 DPI, but realistically, most pro players set their mouse sensitivity below 1000 DPI. So, if your MMO mouse supports high DPI, which is quite likely as 2000 DPI is the modern-day standard, an MMO mouse will do great in an FPS game as well. Moreover, the extra buttons will also come in handy for the fast-paced action in an FPS title as you can assign them to reload, heal or scout.