Thanks, folks! You’ve convinced me that it’s fine. Now I have no excuse for not cleaning the darn thing.
Another gentle product is Bar Keepers Friend. As the name implies it’s good for glassware. I buy it in the grocery. It’s next to the other, more abrasive scouring powder cleaners.
I’d trying baking soda first. Then try Bar Keepers Friend if your Pyrex is still stained. Be gentle and don’t rub really hard. I’ve used this stuff on Corning Ware pans and they weren’t damaged.
What sort of sponge/cloth do you use on Corningware with the Bar Keeper’s Friend?
I normally use a nylon pad sponge.
https://www.walmart.com/ip/Lola-Nylon-Sponge-Cleaning-Pad-Pack-of-3/167618856
Or just a regular thin kitchen sponge.
Don’t rub too hard. Just a gentle wipe with the sponge.
Hmm. Well, I’ll give it a try!