I recently looked up online how to clean oven racks, and the suggetions that I found were to place a couple of old bath towels in the bottom of your bathtub, put the racks on top of the towels. Then cover with hottest water possible from tap. Next add either 1/2 cup of dishwasher detergent, laundry detergent, dishwashing detergent (like Dawn) or baking soda and vinegar. Let racks soak overnight, and each method promises that the baked on grease will come off easily. One person even suggested adding 6 to 8 fabric softner sheets to the hot water and detergent. She claimed that this really loosend the baked on grease. I would imagine that any of these methods would be safe in a fiberglass tub, but I would probably stay away from using dishwasher detergent. I would also recommend that you be sure to keep the tub covered with the towels, as the racks themselves could easily scratch the tub. I haven't personally tried this method yet, but I plan to when the racks on my new oven need cleaning.