using_an_index.js

/***************************************************
 * Assuming you can't use priorities (e.g. they are already being used for something else)
 * You can store the email addresses in an index and use that to match them to user ids
 ***************************************************/
 
var fb = new Firebase(URL);

/**
 * Looks up a user id by email address and invokes callback with the id or null if not found
 */
function getUserIdByEmail( emailAddress, callback ) {
   fb.child('emails_to_ids/'+emailToKey(emailAddress)).once('value', function(snap) {
       callback( snap.val() );
   });
}

/**
 * Creates a new user record and also updates the index
 */
function createNewUser( userRecord ) {
   var id = fb.child('user').push(userRecord).name();
   fb.child('emails_to_ids/'+userRecord.email).set(id);
   return id;
}

/**
 * Firebase keys cannot have a period (.) in them, so this converts the emails to valid keys
 */
function emailToKey(emailAddress) {
   return emailAddress.replace('.', ',');
}

using_priority.js

/***************************************************
 * You can set the priority on the user records to the email address, then use that to look them up.
 * Assumes user records are stored in the /user path
 ***************************************************/
 
var fb = new Firebase(URL);

/**
 * @param {string} emailAddress
 */
function findUsersMatchingEmail( emailAddress, callback ) {
    fb.child('user').startAt(emailAddress).endAt(emailAddress).once('value', function(snap) {
        // the keys are the user ids, the values are objects containing each user record that matched (presumably 1?)
        callback( snap.val() || {} );
    });
}