So you think it's silly to not want to introduce new potentially remotely-exploitable CVEs in one of the most important pieces of software (the web browser) on one's computer? Or are you implying those 100k lines of multithreaded C++ code are bug-free and won't introduce any new CVEs?
Multiple severe attacks on browsers over the years have targeted image decoders. Requiring an implementation in a memory safe language seems very reasonable to me, and makes me feel better about using FF.