The wattage of a fridge bulb does not matter because the light is only on when you open the door and it will cool down very quickly, thus extremely unlikely to affect the temperature in your fridge. You let in far more heat just by opening the door. And LEDs are FAR more efficient that even the best incandescent bulbs. And mercury should almost never be a factor unless you plan to shatter the bulb while it's inside your fridge.
You can test this very easily. Just put a 60 watt bulb in a socket and turn it on. Do the same with an LED of comparative lumens (15 watt). Leave them both on for 10 minutes. Now hold your finger to the incandescent bulb - hint, you can't. Now touch any part of the LED - hint, it will be warm, but not uncomfortably so. You may be thinking about high wattage LEDs. Those emit far more light and can get very hot, but keep in mind that a comparable incandescent would be hot enough to boil water. What makes LEDs so much more efficient is that they turn more of the electricity to light than heat compared to incandescent and fluorescent bulbs.