The last time this was posted I hacked up a quick and dirty Python client for Send: https://github.com/nneonneo/ffsend. I just updated it for recent Send changes, which streamlined the crypto and removed some redundancy.

Firefox’s JS client requires the whole file be in memory in order to perform the encryption, and has to decrypt the whole file in memory in order to download it. My client doesn’t have that limitation so it could theoretically upload much larger files (subject only to the server’s upload limit).