Doorspawn: C++ definitely has it's place as a programming language, but when designing plugins for a non-high-performance application like a web browser, idiot-proofing is a good thing. Most add ons shouldn't be doing anything 99% of the time anyways, so any ***potential*** performance hit isn't important.
Besides, there are very few C++ applications that are completely bug free, and if there is a sand box is doing some memory management and blocking access to certain standard library functions, then you might as well have started with a managed language with it's own libraries, because you're incurring just as much of a performance hit, with potentially less safe and stable code.