well i have faith that it will be superior than C++
C++ is just a standard (im not into programming) but C++ doesnt look rock solid to me and it has many holes when it comes to something as a browser, so if they write the whole firefox in Rust, it will probably better compared to the current C++ firefox
it will probably be more responsive, faster, less a memory hog
in a few words: im looking forward to it, firefox is more like it used to be called – Firebird (Phonex) it will be revived from its ashes
as firefox 4.0 is just an improved version of 3.6 .. probably some drastic, but still small improvements
firefox rust will be a completely new experience, heed my words