There is no way to set the fonts back to defaults.
As I noted on IRC, a "Reset to Defaults" is an extremely heavy-handed approach that is a little too "all or nothing" for my liking. Our Fonts UI isn't exactly something I'm happy with, but this isn't the answer.
sorry for bugspam, long-overdue mass reassign of ancient QA contact bugs, filter on "beltznerLovesGoats" to get rid of this mass change
Add "reset all fonts" button. The Preferences > Fonts is a giant tree. It needs a [ ]Reset all fonts button, else the only way for the user to reset all of them is to reset all of firefox. 27.0a2 (2013-12-07)
Adding a Reset all Fonts button there would take staff a minute to make, and save the user hours of recursively checking every nook an cranny for any non-Default settings... which by the way he has no way of figuring out if they are default or not, without comparing a new employee in the neighboring cubicle's Firefox, each language at a time!