To unlock this achievement, you must rescue 17 cats and extinguish 17 BBQ fires across the city. You must be using a firefighter disguise or character to complete both of them.
For the cats, you must find the area they are in (which is generally up in a high spot), and interact with the action circle, where the cat will jump down.
For the BBQ fires, you must find a red and black grill with fire on it and spray it with your water gun.
You will earn a gold brick for each individual challenge completed, and you also get a character token for completing all 17 of either challenges.
You can use the map and highlight a specific district in order to tell what challenges you have completed, and you can also use the Data Scan Red Bricks in order to locate the challenges if you are struggling to find them.
Down below are video guides for the challenges by packattack04082.
Rescuing 17 cats:
Extinguishing 17 BBQ fires: