tmpfs is a filesystem residing in RAM only.

tmpfs can be used to speed frequent I/O ops, reduce disk wear or ensure content removal after shutdown.

You can use it to, for example, speed up Firefox by moving its profile directory into a tmpfs. Doing the same to its cache is not recommended as it can cause loss of data. Therefore, you can disable it entirely within Firefo by setting browser.cache.disk.enable to false under about:config.