In this article, we present FireSim, an open-source simulation platform that enables fast cycle-exact microarchitectural simulation of large scale-out clusters by combining FPGA-accelerated simulation of silicon-proven RTL designs with scalable, distributed network simulation, running on a public-cloud host platform. By introducing automation and harnessing cloud FPGAs, FireSim provides the usability and productivity of software full-system simulators with the high performance and accuracy of FPGA-accelerated simulation, while adding the unprecedented ability to scale to globally cycle-accurate simulations of thousands of networked nodes. To demonstrate FireSim's scalability, we automatically generate and deploy a target cluster simulation of 1024 3.2-GHz quad-core server nodes, each with 16 GB of DRAM, interconnected by a 200 Gb/s network with low latency, which simulates at a 6.6-MHz processor clock rate («500× slowdown over real time). In aggregate, this simulation harnesses millions of dollars of FPGAsat a cost of only hundreds of dollars per simulation-hour to users.