You can change the font default by changing the Content.css file found in assets\scripts\tiny_mce\themes\advanced\skins\default folder. Within this file you will need to edit the line below which is located at the beginning of the css script. Edit the highlighted area to the font you wish to set as your default.

body, td, pre {color:#000; font-family: Arial; font-size:12px; margin:8px;}