Without knowing almost anything about Firefox Send, it does seem that the key is embedded in the link you give to your friends. In that case E2E means that the key is not stored on the server. In order to guarantee that, the link is probably in two parts, one that identifies the file on the server and one is the key. The key part of the link is probably generated on your machine and thus never sent to the server (to prevent it being in any logs or what not). So if Mozilla's servers are compromised the attacker still would not be able to decrypt your files. Of course if the server is compromised the attacker could serve up malicious JS for future uploads.