C++ is not Ocaml, and Mozilla is not random-opensource-project. When you're contributing to an established project, you follow the conventions there even if they aren't what you'd consider ideal.

Brendan Eich even said as much in the comments of the bug: he'd rather be programming in a pure, expression-based manner too, but C++ is not Ocaml. And for a lot of C++ code, no-else-after-return works reasonably well.