As far as I know, even tracking files is not really really secure.
Files get (physically) written here and there... they get out of control.
If it's really that sensitive to you, better mount /home/username/.mozilla directly as an encrypted filesystem (e.g. encfs or ecryptfs).
Instead of deleting, what you will do is you will create a brand new filesystem with a brand new encryption key, copy the current profile from a mounted
with the sqlite file, and securely delete the encryption key of the previous filesystem. Without the key then it's really garanteed never to be recovered.
(I'm assuming Firefox really enforces that this database file will not contain traces of the stuff you have deleted.)
Sorry, your question is about a user-friendly approach, and my answer is that even the non-user-friendly methods may be unsecure depending on what level of security you need.