We have some details on difficulty tuning coming with this week's maintenance in Season of Discovery, with specific changes coming to individual Heat levels and bosses!
Molten Core (Source)
We’re working on a series of hotfixes to adjust Molten Core, and we don’t expect to get these changes implemented until some hours have passed after weekly maintenance tomorrow morning.
Ragnaros no longer submerges twice on Heat 1. Baron Geddon now casts Armageddon at 5% on Heat 1 (was 10%). Baron Geddon now casts 2 Living Bombs at a time on Heat 2 (was 3 casts). Living Bomb no longer leaves Living Fallout pools on Heat 1. Flamewaker Protectors now cast Dominante Mind much less frequently. We’re maximized the cast and recast times of several dispel mechanics. We’ll let you know when these changes go live tomorrow afternoon.

Blizzard have announced there was an issue with non-boss mobs in Molten Core not having the correct immunities in the first few minutes of the raid's launch. A hotfix has been deployed, but players that entered the raid in that period can also resolve the issue by soft resetting the instance or switching Hear level. 
Molten Core (Source)
About 5 minutes after Molten Core opened, we deployed a hotfix to correct immunities on non-boss enemies. Players who zoned in during the first few minutes can expect to find boss adds that have unexpected immunities.
If you wish to, a soft reset of the instance should result in the non-boss enemies spawning correctly. Also, quickly swapping to a different Heat level and back to your original Heat level at the Hydraxian NPC near the entrance will reset the boss add immunities without needing an instance reset.
Thank you!

The new Molten Core and Onyxia's Lair are now live in SoD Phase 4! 
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It's time to put on your fire resist gear to face Onyxia in her lair and Ragnaros, the fire lord, in the Molten Core.
World Bosses Now Live
Experience Lord Kazzak and Azuregos as brand-new instanced versions of these encounters. These encounters can be experienced with up to 40 players but will be tuned as 20-player content, making them both pick-up-group-friendly with 40 players, but also giving players the option to maximize their rewards by attempting them with 20-players.
Onyxia's Lair and Molten Core Now Live

Experience Molten core as a 20-player dungeon with 10 classic bosses (+1,) revamped loot, redesigned Tier 1 sets, and more. Players will also be able to turn up the heat with a new variable difficulty mechanic allowing players to earn more rewards from undertaking a higher difficulty. Don’t forget your fire resist gear! You are going to need it. We’ll be adding one more surprise to this classic raid dungeon for intrepid adventurers to discover.
Onyxia will also be adjusted to accommodate 40 or fewer players along with some additional loot adjustments.
New Raid Lockout interval
Raid lockouts for Season of Discovery Phase 4 raids will occur regularly once a week for Molten Core and two times a week on static reset days for Onyxia’s Lair, Lord Kazzak, and Azuregos. Players in North America will have their resets for these bosses on Tuesdays and Saturdays, in Europe, on Wednesdays and Sundays, and in Asia on Thursdays and Mondays.
Gather your allies and go forth to battle!

Blizzard will be increasing the drop rate for Librams of Resilience in BRD and BRS, as well as Burning Essence rates from the Dark Coffer with a hotfix today. They also explained some of the fire resistance numbers found in today's Molten Core post, pointing out that those listed were fully buffed.
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We agree, actually. We are sending a hotfix very soon to add a chance for Librams of Resilence to drop from certain Blackrock Depths and Blackrock Spire bosses, and increase the liklihood you get Burning Essence from the Dark Coffer in Blackrock Depths. This should help make these items a bit more available, but still require the same process and ceremony to complete the arcanum.
I want to also take this opportunity to also clarify that the fire resistance numbers listed in the blog post are fully buffed. So that ~100 fire resistance breakpoint for Blistering/Heat 2 only requires you to have ~40 unbuffed if you have a shaman or paladin in your group. For many classes that is a single item and the new +20 Fire Resist cloak enchant from Hydraxian Waterlords reputation. For Molten/Heat 3 you’ll need to do a bit more prep and that is intended, but as alluded in my previous post, Heat 2 should be very approachable for almost anyone.

With the addition of World Boss instances raids, it seems Season of Discovery has run into a hiccup, as players were unable to properly resurrect! Blizzard are working on the issues and as a temporary fix, players will be respawning at the Westfall/Barrens graveyards before showing up at the intended one momentarily. 
Resurrections (Source)
We’re currently working to address an issue with resurrection. For the time being:
When dying and releasing inside of each raid, you will momentarily be ported to Westfall or the Barrens before being transported to the correct graveyard. Thank you for your patience!