Logged in this weekend to do my weekend WZFF grind only to not see Mythic WZFF in the playlist section! If anything we need the Mythic variant to be a primary playlist, over something like the Heroic playlist, to help all of us grinders to get to 152 before the Infinite launch. Is there any announcement or any reason why Mythic is not present?
Just soo you know Mythic Firefight isn't actually a very fast way to get XP unless you have a full team and can finish the games quickly. Unless your playing it for the challenge then stick with heroic or legendary. Mythic does pay more XP but that's only if you finish all the rounds and the games generally are much longer.
Heroic will give you about 5000 XP without a boost and games are around 13 minutes on average. Legendary will give you about 6000 XP without a boost and the games are around 15 minutes on average. Mythic will give you about 7000 XP and the games are usually about 20 minutes.
Keep in mind if you have a team the times of each difficulty can be different but they are averages based on if you are playing solo. I saw you mentioned you wanted to grind to 152 soo I thought would give you the XP numbers