Pyrex glass round casserole safety in high temperature
Update based on edited question:  there are no issues of toxicity.  



It is a very poor idea to use glass cookware on a burner.  Not all Pyrex is made from high quality borosilicate glass anymore, and even if you have some, the issue is thermal shock, not toxicity.

If you heat or cool glass very rapidly, the internal stress caused by thermal expansion (or contraction, when cooling) can cause it to shatter.  This can be a real safety issue when it is full of hot food.

For this reason, you generally only want to use glass cookware in the oven, and never transfer it directly from the freezer to the oven, or the oven to the freezer.