Simple cache mechanism?
From a once over;


Unless YFeX is also a constructor, it should be yFex
lowerCamelCase is good for you: account_modal -> accountModal
I would store YFeX.cache.account_modal into a var cachedAccountModal so that you do not have to repeat YFeX.cache.account_modal all the time
Your code is very optimistic, you should also deal with fail
Consider using the shorthand $.get()


Other than that, the code looks fine to me.