Browser notifications are nice, but most browser support still requires the web site to be open. It's good to see that Firefox is implementing the W3C standard, but like with many new features, there is often a fair amount of time that has to go by before the standard is adopted in practice. And of course, people have to upgrade their browsers to make use of it. We're unlikely to include a feature that is not yet widely supported by multiple major browsers. Even if we do offer some desktop notifications (we have discussed this), it would be completely optional.