memoir has at least one package that modifies the output routine and conflicts with
footbib, does anyone know which one(s)?
(Derived from the comments of daleif)
memoir includes the functionality of various packages, and tells LaTeX that these have been emulated and thus LaTeX should not load them if the user asks for them. With regard to the OP's question:
memoir itself changes the output routine in various ways.
\DisemulatePackage only allows the respective package to be loaded -- the command cannot undo changes to the output routine that
memoir has already effected; therefore, one cannot get rid of a conflict between, e.g.,
memoir stemming from their modifications to the output routine.