FTR, three good reasons to use `mozL10n.localize` on all localizable HTML elements:
* it works for all the tricky cases where setting the element textContent is not enough;
* it works asynchronously: you don’t have to wait for the `localized' event before using it;
* it sets the data-l10n-* attributes, which ensures that the HTML element will be properly updated if the language is changed.