This is not how the l10n magic works. I guess you’re used to gettext (which makes a lot of sense but hasn’t been accepted by our l10n team). ;-)

The `_()` helper takes an l10n-id as argument, not a human-readable string, so it would be something like:
```javascript
if (state == 'verified') {
  menuDesc.innerHTML = _('fxa-logged-in-text', Normalizer.escapeHTML(email));
} else if (state == 'unverified') {
  menuDesc.innerHTML = _('fxa-check-email');
} else { // state == 'loggedout'
  menuDesc.innerHTML = '';
}
```
where these two l10n entities are defined like this:
```ini
fxa-logged-in-text = Logged in as <span id="fxa-logged-in-email" class="fxa-email">{{email}}</span>
fxa-check-email = Please check your email.
```

Beware, this means you’ll delete and recreate the `span#fxa-logged-in-email` element, which is a good reason for not storing it in a global variable (see `loggedInEmail`, line 177).

However, I’d strongly recommend create this `span#fxa-logged-in-email` element dynamically and rely on the `localize()` helper, like that (untested):
```javascript
if (state == 'verified') {
  var emailElement = document.createElement('span');
  emailElement.id = 'fxa-logged-in-email'; // in case you really need it (e.g. for the tests)
  emailElement.className = 'fxa-email';
  emailElement.textContent = Normalizer.escapeHTML(email));
  localize(menuDesc, 'fxa-logged-in-text', { email: emailElement.innerHTML });
} else if (state == 'unverified') {
  localize(menuDesc, 'fxa-check-email');
} else { // state == 'loggedout'
  menuDesc.innerHTML = '';
}
```
This is more verbose but the strings will be properly updated if the language is changed, and the l10n entities will be much easier to handle by our l10n contributors:
```ini
fxa-logged-in-text = Logged in as {{email}}.
fxa-check-email = Please check your email.
```

While I’m at it, two punctuation nits regarding l10n strings in general:
* don’t forget the trailing dot when it applies;
* please use curly quotes `’` instead of straight quotes `'`.