I believe the FE explosion is half fire damage, and half physical damage (like CE). It's also based on the monsters Hit Points, so it scales up with the more players that enter the game.
It's not too bad in Norm. They (blizzard) toned it down in Hell so it's not quite as horrible, but still dangerous. In NM it's at full strength and completely lethal, 95% fire resist or not.
If a monster has 10,000 HP (which I don't think is unreasonable in games with more than 5 players), and the FE pop does 60% of that monsters damage, it will do you for 6,000 damage (3,000 fire, and 3,000 physical).
With 95% fire resist you'd take 150 fire damage, which is certainly reasonable. However, you'd also take 3,000 physical damage as well... deeds.
Now add Amp damage into that equation, 6,000 physical damage... deeds.
Now try it on a monster with lots of hit points, like NM lister in a full game, or Hell Ancients in a 3 or 4 player game... deeds.
There is also Conviction and Lower Resist to worry about too. You can try tanking them if you want, but I think "if you play with fire long enough you'll get burned."
Note: The above numbers are guesses, and only intended to help illustrate my point.
-hps