I really didn't like the Firewalker missions. You couldn't save manually while in the hammerhead so you were at the mercy of checkpoints in the levels, enemies could destroy the vehicle rather quickly if you weren't careful and the game thought it would be a good idea to have platformer sections over lava that almost instantly destroy the vehicle.
Completed Firewalker for the first time a few days back and it was pretty meh. Filler content.
I can confirm all the DLC I've listed will add to the readiness level to fight the Reapers. Not a super ton, but every little bit helps when going for the perfect ending which I hear has quit the lofty number to reach. Yes, even bring down the sky.
Hell, all I care is that humans, Asari, Turians and Salarians make it. Rachni I always destroy no matter what, Krogan I'd first throw into the grinder and Batarians are just useless.
In fact if there is a way to direct the Repers to mow down the Krogan and Batarians, that will be one hell of a good ending for me.