Actually, the main thing that (I believe) makes the FF codebase sometimes hard to read is that it had to work on a number of platforms where C++ compilers were

Actually, the main thing that (I believe) makes the FF codebase sometimes hard to read is that it had to work on a number of platforms where C++ compilers were just completely broken. By now, either these platforms are dead or the compilers have been fixed, and the code of Gecko is improving steadily, but you can't undo 20 years of history that easily. On the plus side, Rust is a really, really nice language :)