It works like this — you upload the file to send.firefox.com, and the file is encrypted. Your recipient receives an email, and clicks the link, and the file and decrypted and downloaded.
Overall, the service provides end-to-end encryption. Here are technical details of how this works:
The client encrypts the file that is uploaded, along with some metadata. The key is appended to the shared URL in the fragment/hash, and is never sent to the remote server. Only people having the URL including the secret will be able to download and decrypt your shared file. See https://github.com/mozilla/send/blob/master/docs/encryption….Tim Visee on Hacker News
It’s worth noting that whoever gets access to the emailed link will be able to view the file. So this transfer relies on the security of your email (which is usually unencrypted).
Firefox Send will also be available as an Android app in beta later this week