I'm familiar with the meaning of MDMA, but is XTC some new street name for "ecstasy"
As far as being synonyms, people generally differentiate between "ecstasy"/"E" as MDMA in pill form and "molly" as MDMA in powder form.
But maybe that's because I grew up in a time where you can send stuff to a lab to confirm what it is.
XTC aka ecstacy is MDMA + additives like speed or caffeine/whatever.