Choosing the right memory allocator
presshell arena nsipresshell::allocateframe() and nsprescontext::allocatefromshell() can be used to allocate memory from an arena maintained by the presshell.
... the corresponding deallocators are nsipresshell::freeframe() and nsprescontext::freetoshell().

nsIContentViewer
prescontext nsprescontextptr read only.

nsIDocShell
obsolete since gecko 1.8 prescontext nsprescontext presentation context for the currently loaded document.