ESA's Far-IR and Sub-millimetre-wave Telescope, FIRST, is an astronomy space mission which will provide an unobstructed view of the universe in the last major unexplored region of the electromagnetic spectrum. The satellite is planned to be launched in mid 2006 and will carry a payload of three instruments spanning the wavelength interval 80 and 800 pm. ESA has recently released an Announcement of Opportunity (AO) asking for interested parties to make proposals to provide these instruments and this paper will describe our proposed front-end for a Heterodyne Instrument for FIRST HIFI. HIFI will be built by a large consortium of European, American and Canadian institutes which began work defining a heterodyne instrument in 1996. HIFI will cover the frequency interval 480 1250 GHz in 5 bands using pairs of SIS tunnel junction mixers to receive both polarisations. The frequency ranges 1600 1900 GHz and 2400 2700 GHz will also be covered using single hot-electron bolometer mixers. A modular construction will be used with 6 mixer assemblies, one for each of the 5 lower bands and one for the two high-frequency bands. Each mixer assembly will contain optics for local oscillator injection and the first stage of IF amplification. The local oscillator signals are generated outside the cryostat in a separate unit and pass through dedicated windows in the cryostat wall. The local oscillator unit is described elsewhere. Low noise InP HEMT's with very low power consumption will be used in the IF preamplifiers and will provide a 4 GHz IF bandwidth. A suite of spectrometers in the warm service module of the spacecraft will analyse the IF signals with frequency resolutions ranging from 100 kHz to 1 MHz. The mixer assemblies slot into a housing containing the optics common to all bands. The common optics performs the functions of refocusing the beam from the telescope, splitting the focal plane amongst the 6 mixer assemblies, chopping, and calibration.