Following a call on social networks, here's a quick list of really missing applications for Firefox OS, from users' point of view.

Security :
SMS / MMS encryption
Tor ability
GPG in mail application
OpenVPN

client Sync :
CardDAV native support (with self-signed TLS certificates)
ownCloud application (with all OC services)

Misc :
SSH client

It might be difficult to create those applications using only HTML5/CSS3, so a low layer integration (Gaia/Gonk) might be necessary. To Firefox OS developers and Mozilla : please listen to the community needs. Thanks a lot !