Accessing OS X built in dictionary lookup in Firefox
As of now (Dec 1 2014), although the option is not present in right-click menu, three-finger dictionary lookup works perfectly fine in my Firefox (Developer Edition 35)

Also there have been certain plugins which help you look up words in dictionary through right click. For example https://addons.mozilla.org/en-US/firefox/addon/look-up-in-dictionary-7261/

EDIT (Oct 2017): I'm now using the new e10s Firefox and the dictionary lookup still works. You don't need any plugin, just tap with three fingers or press Control + Cmd + D.