RemoteAccessibleBase::mParent is currently an id. This means we have to do a hash table lookup to get a parent. While this doesn't seem like a big deal, we do a lot of ancestor walks and sometimes tree searches. Making it a pointer would make this faster.
We have to be careful here because mParent can refer to an OuterDoc Accessible in another document, which might be why this was made an id in the first place. That said, we already have this problem with mChildren, so I don't think this is any less safe than that.
I think I've seen this show up in profiles, but I don't recall where now. :( It's probably not worth tackling this unless it shows up again, but I thought this worth filing while it's on my mind.