My mom has told me that because I used glassware for baking fish that It can no longer be used for anything else. Is there truth to this or myth?
Hmm..I hate to disagree with someone's Mom, but I have not found that to be true. Glass is not absorbent.
I call bullshit. If its washed its fair game for everything.
Only if you have to keep Kosher. Beyond that use what ever tool works best for preparing your dish.
Unless it's for religious belief all dishes can be used for any purpose in your kitchen. Otherwise you would have like 5 sets of bakeware 4 of which would serve no purpose.
Actually, even in kosher cooking, fish can go either way...it's neutral, can be eaten with meat, dairy, whatever. (I think your mom might have been thinking that fish flavors are absorbed by the cookware - which won't happen with glass.)
You should be able to use it for more than just fish, after washing. Even for kashrut reasons (which I don't think is the reason here), it would without question be usable for both. Pyrex (or other glass) cleans up well.