  The nonlinear evolution of Alfv\'enic fluctuations in the firehose unstable
regime is investigated numerically and theoretically for an anisotropic plasma
described by the one-fluid double adiabatic equations. We revisit the
traditional theory of the instability and examine the nonlinear saturation
mechanism, showing that it corresponds to evolution towards states that
minimize an appropriate energy functional. We demonstrate that such states
correspond to broadband magnetic and velocity field fluctuations with an
overall constant magnitude of the magnetic field. These nonlinear states
provide a basin of attraction for the long-term nonlinear evolution of the
instability, a self-organization process that may play a role in maintaining
the constant-$B$ Alfv\'enic states seen in the solar wind in the high-$\beta$
regime.
