Combine fire-roasted tomatoes and adobo sauce in a stockpot or Dutch oven and cook on medium-low for a few minutes, stirring occasionally.
Gently stir in the rest of the ingredients and let simmer for a few more minutes.
Add the chicken breasts and turn to make sure they are all evenly coated.
Cover and cook for 45-50 minutes, or until chicken is cooked through and no longer pink.
Remove the chicken and place on a plate to cool.
Using an emulsifier, blend the remaining sauce and juices left over in the pan to create a smooth sauce.
When the chicken is cool enough to handle, shred it and put it in a bowl.
Pour the sauce mixture over the chicken and toss to coat.