A firefighting foam-mixing system, comprising: - a water line for providing a water flow, - a storage tank having an inlet and an outlet, the storage tank comprising an internally arranged membrane adapted for holding a foaming agent and connected to the outlet, - a first conduit having a first end connected to the inlet of the storage tank, and - a second conduit having a first end connected to the outlet of the storage tank and a second end connected to the water line at a first position, wherein: - the firefighting foam-mixing system further comprises a water diverter arrangement connected to the water line at a second position and to a second end of the first conduit, the second position being upstream of the first position in relation to the water flow through the water line, - the water diverter arrangement is configured to, independent of a current flow of water at the second position, divert a predetermined proportion of the water flowing through the water line at the second position, to the first conduit for pressurizing an outside surface of the membrane, the water diverter arrangement is a proportioning valve, and the water diverter arrangement comprises: a housing having a water inlet side, a water outlet side, and a water diversion port, a spring-mounted plate arranged on the inlet side of the housing and connected to a spring, the spring being arranged to provide an opposing force in comparison to a current pressure of the water through the water line, a piston connected to the spring-mounted plate, and the piston being configured to control an opening at the water diversion port through which the predetermined proportion of the water is diverted. - a water line for providing a water flow, - a storage tank having an inlet and an outlet, the storage tank comprising an internally arranged membrane adapted for holding a foaming agent and connected to the outlet, - a first conduit having a first end connected to the inlet of the storage tank, and - a second conduit having a first end connected to the outlet of the storage tank and a second end connected to the water line at a first position, - the firefighting foam-mixing system further comprises a water diverter arrangement connected to the water line at a second position and to a second end of the first conduit, the second position being upstream of the first position in relation to the water flow through the water line, - the water diverter arrangement is configured to, independent of a current flow of water at the second position, divert a predetermined proportion of the water flowing through the water line at the second position, to the first conduit for pressurizing an outside surface of the membrane, the water diverter arrangement is a proportioning valve, and the water diverter arrangement comprises: a housing having a water inlet side, a water outlet side, and a water diversion port, a spring-mounted plate arranged on the inlet side of the housing and connected to a spring, the spring being arranged to provide an opposing force in comparison to a current pressure of the water through the water line, a piston connected to the spring-mounted plate, and the piston being configured to control an opening at the water diversion port through which the predetermined proportion of the water is diverted. the water diverter arrangement is a proportioning valve, and the water diverter arrangement comprises: a housing having a water inlet side, a water outlet side, and a water diversion port, a spring-mounted plate arranged on the inlet side of the housing and connected to a spring, the spring being arranged to provide an opposing force in comparison to a current pressure of the water through the water line, a piston connected to the spring-mounted plate, and the piston being configured to control an opening at the water diversion port through which the predetermined proportion of the water is diverted. a housing having a water inlet side, a water outlet side, and a water diversion port, a spring-mounted plate arranged on the inlet side of the housing and connected to a spring, the spring being arranged to provide an opposing force in comparison to a current pressure of the water through the water line, a piston connected to the spring-mounted plate, and the piston being configured to control an opening at the water diversion port through which the predetermined proportion of the water is diverted.