Your Firra Casserole Dish is designed to last. Over the years, the internal coating could develop some marks and can take on the colours of what you're cooking. This is perfectly normal and nothing of concern.
For removing extra stubborn baked-on residues
Avoid harsh, abrasive cleaners and scrubbing pads that could damage the enamel coating. A simple three-to-one mix of water and baking soda left in your Ambrosia overnight and a rinse and wipe with a soft cloth the next day should do the trick. Then, just make sure your dish and lid are thoroughly dried before storing.