Bug 1402174 made all arenas registered in a Red-Black tree. Which means they are iterable through that tree, making the arenas list now redundant. The list is also inconvenient, since it needs to be constantly reallocated, and the allocator in charge of the list doesn't know how to free things. Iteration of arenas is not on any hot path anyways, so even though iterating the RB tree is slower, it doesn't matter. So we remove the arenas list, and keep a direct pointer to the main arena for convenience (instead of calling First() on the RB tree every time) Review commit: https://reviewboard.mozilla.org/r/184430/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/184430/