Yeah Firefight is too easy on matchmaking, it's only through selecting a custom firefight game to play with friends that you can pick a 'Firefight Classic' game which is more challenging like ODST Firefight.
From what I read on the Bungie forums, there will be a playlist update next month, so they might change what is on there. Also, there's been news that they have been banning people for credit farming, as supposedly you can make a ton of credits by playing Gruntpocalypse over and over.
I actually finished the campaign last night.
*SPOILERS BELOW*
While I enjoyed the game very much, it was very similar to ODST in that everything is setup around multiplayer. All the maps available in multiplayer, aside from the Forge World ones are single player campaign levels. It makes you wonder which way around they decided to make them...
As for the story, it was good, but there were times I thought they could have lingered on things to make it more dramatic and fleshed out the story a bit more. It's all done generally quite quickly.
With the ending, I've noticed other people also pointed out that they didn't totally think it through. There is a bit where a Pelican shows up to drop off some marines... surely it would have made sense to have just jumped on that to get to the Autumn, rather than fighting it out for another 10 minutes before Keyes shows up in his. I also thought it was a shame they didn't have Johnson do a cameo, he could have been on Keyes Pelican with him.
I was going to say that it was also a shame that MC didn't have a cameo, but then story wise, he is supposed to be asleep. However, I just found out that he does actually have a cameo! During one of the cutscenes of the Autumn's docking bay, with the Warthogs, you can actually move the camera and see MC asleep in his pod! Nice little Easter Egg!