Thanks for the response, Jeffrey.
And thanks for checking in with your issue, Linda. I fully agree with Jeffrey’s response – please try all of those steps, and if the issue still persists, than definitely let me know and I can look into it.
With the upgrade to WP 3.9 came a pretty serious upgrade in the TinyMCE visual editor that created a lot of problems for a lot of people:
https://www.google.com/search?q=wordpress+3.9+visual+editor+&oq=wordpress+3.9+visual+editor+&aqs=chrome..69i57j69i64l3.5989j0j7&sourceid=chrome&es_sm=91&ie=UTF-8
Generally, this problem stems from plugins that add buttons, drop-downs, and other controls in the visual editor, and do say using a method that was supported in the older version of TinyMCE but break in the new version (4.x). Unfortunately, there’s not much to be done other than disabling the plugin and notifying the author (please do this to make sure they know and have a chance to provide a fix).
I can tell you that I’ve tested Better Font Awesome with WP 3.9 and it’s working just fine for me (and is for Jeffrey too, I believe). So bottom line, try disabling your plugins one by one to see which one(s) is the culprit.
Best of luck, and let me know if I can be of any further help.