Tools - Options - General -
"Fonts and Colors."
That's how to change it BACK. Unfortunately I don't know what shortcut key you typed in to change it, so I don't know what it's changed TO, so you'll have just have to change it back to the Firefox defaults.
Mine has these listed in this order, and it's the defaults:
Serif
Times New Roman
Arial
Courier New
KeyserSoze