dude, Firefly doesn't need to be designed around; it's not a disadvantage at all. It really does need SOME kind of nerf, and a notable nerf, and even then it's still a big advantage. Consider that the Dungeon does not really have MT OHKOs until F6 or so, and enemy ST offense that ramps over 2 PCHP in round 1 is fairly unusual. While by default it should go on a low-value yet moderately tanky member of your party, it's absolutely fine to stick it on DW1 Hero or whatever. So suppose it is on DW1 Hero - that's still 1 PCHP of durability to chew through, and THEN the enemy can worry about going for the reviver. And if there's 2 revivers they may need even more offense to actually get anywhere.
I'm not arguing that a speed hit won't hurt, because it will. But Firefly is still totally awesome because the Dungeon is rocket tag with most fights having turned by the end of round 1, and you're forcing the enemy team to spend round 1 very inefficiently. That is still amazing, and frankly would not make THAT many fights in Firefly teams that have already gone through the Dungeon change all that much. (Your old team, for example, would still require the extremely specific spoiler to W8 in badass MT physicals to really lose - speed is just not a huge concern so long as an MT OHKO or MT status can be avoided.)