Answer:
Depending on the type of fabric used to construct the balloon the internal temperature limit will be between 125oC and 150oC / 250oF and 300oF. Nylon fabric balloon have the lower limit, polyester balloons generally have the higher limit.
The above are the flight manual limits. On a typical flight a balloon would be load so that the internal tempewrature was not above 100oC / 212oF