Low level software is often granted high privilege, yet this need not be the case. Although vendor firmware plays a critical role in the operation and management of the machine, most of its functionality does not require unfettered access to security critical software and data. In this paper we demonstrate that vendor firmware can be safely and efficiently deprivileged, decoupling its functionality from isolation enforcement. We introduce a new class of systems, called virtual firmware monitors, that run unmodified vendor firmware in userspace through software-based virtualization of the highest privilege mode of the application CPU. We describe the implementation of Miralis, a RISC-V virtual firmware monitor, and develop three security policies to protect the OS, enclaves, and confidential VMs from malicious firmware. We verify key components of Miralis, such as instruction emulation and memory protection, through exhaustive symbolic execution. Finally, we demonstrate that Miralis can effectively virtualize unmodified vendor firmware for two hardware platforms with no performance degradation compared to native execution.