Instead of "getting rid of C++" in a project written almost exclusively in C++, it would seem more sensible for Google to just wait for Firefox to be completely rewritten in Rust in the next ~10 years and if it's really that much better, fork it, add some googley stuff on top like user-tracking and mandatory log-in and then release it as the new Chrome.
Alternatively, since Rust natively interops with C and C++ pretty straightforwardly, it should be possible to write new components in something like Rust and opportunistically rewrite old components on an as-needed basis.