FYI 343 in there infinite wisdom decided to decrease the amount of exp of mythic firefight it used to payout 25k exp With a legendary boost and now gives 19k with a legendary boost. Heroic war zone firefight gives 17k with a legendary boost and takes around 5 minutes less to complete.
Old Mythic exp: 25,000 With boost avg time 22
New Mythic exp: 19,000 With boost avg time 22
Heroic exp: 17,000 with boost avg time 17
these are my numbers so they may be slightly differ from yours but for the difficulty jump from heroic to mythic only being 2k exp it’s not worth it.