tmpfs is a virtual, RAM-backed filesystem. It’s lightning-fast, but since it’s RAM-backed, any file written to tmpfs uses precious memory while it’s there, and the entire contents of the virtual partition are lost on shutdown or crash. The good news is that these detriments can be minimized, making tmpfs a viable choice for your profile directory. This document gives some tips on how to mount your Firefox profile in a tmpfs partition while minimizing the downsides of tmpfs.