Nothing in mozilla-central uses the NS_APP_BOOKMARKS_50_FILE/BMarks key except
tests. It's also not been very useful since the switch to places in ... Firefox
3? so even if addons use it, it's not doing them much good.
BookmarkHTMLUtils.defaultPath returns the same thing anyways for those that
really do insist on getting the equivalent thing (but I'd argue
BookmarkHTMLUtils.defaultPath, as well as the browser.bookmarks.file pref could
just go away as well).

Seamonkey does use NS_APP_BOOKMARKS_50_FILE/BMarks, but they really only need
the #define, which they can add in their source.