Yes, but consider the engineering effort to create a general purpose memory safe language vs just a domain specific one in comparison to the work required to write a browser, which also includes another domain specific language (javascript).
Seat of the pants, I would say getting something like rust on par with g++/clang code generation, and 3rd party tools is probably in the same ballpark engineering wise as actually maintaining the browser.
And if you spend a little time looking at firefox, there is/was a lot of low hanging fruit. I've mentioned elsewhere the difficultly building it, but even more than that, is that pretty much every single version of gcc that came out for a while would break firefox in some fundamental ways. And overwhelmingly these breaks were caused by crappy C++ programming where people were doing things that were known not to be syntactically correct but no one bothered to fix them. Then frequently instead of actually fixing them they did things like switch from building with an old GCC to a newer clang because it threw fewer errors. Then they claimed that the result was "faster" than gcc, despite the fact they were comparing an old version of GCC with a newer version clang. When the work was done to actually get it to build with gcc, it turned out to be even faster. (not that gcc is better than clang, only that ideally a project like firefox would compile cleanly with a wide range of compilers). Said compiler warnings are frequently valid, and analyzing them at least to know if its true is a worthwhile code quality exercise.