I had a French bulldog that had very soft poop for a long time and she never got over it so we took her to the vet and they didn't know why it was like that. So after talking to a few different people we tried giving her canned pumpkin. The kind that you make pie with. I believe it was a tablespoon a day anyway after about a week of giving her the pumpkin she had hard poop. I'm not sure why how this works but you could try it. I would ask your vet about it first though and see what he says.
I only feed my dog dry dog food, no human food, scraps or otherwise and his is always firm. I don't recommend this brand in any way, but Purina Pro Plan (savor) is what I give my dog at the recommendation of my vet.
Your dog is suffering from mild diarrhea and your first reaction should be to take it to a vet especially if the diarrhea is intermittent. If you have recently changed your dog's diet, revert to the old diet because the new foods are the likely cause of this diarrhea. Meanwhile give the dog Pedialyte, a supplemental electrolyte solution and it will ease the diarrhea.
If you only feed your dog canned food, start giving your dog kibbles, or a well-balanced, vitamin enriched dry dog food... If your dogs feces are still soft after a couple of days, call your vet. Questions are free-of-charge...