  The frustrated spin-1/2 chain with weakly anisotropic ferromagnetic
nearest-neighbor and antiferromagnetic next-nearest-neighbor exchanges is
studied. We focus on the excitation spectrum and the low-temperature
thermodynamics in the ferromagnetic region of the ground state phase diagram.
It is shown that the excitation spectrum of the model is characterized by the
existence of the multi-magnon bound states. These excitations determine the
low-temperature magnetic susceptibility. The energy of the bound magnon
complexes is found using the scaling estimates of the perturbation theory and
numerical calculations. The relation of the considered model to the
edge-sharing cuprate $Li_2CuO_4$ is discussed.
