Quoting “Firefox 6 ships, but we shouldn't really pay attention” by Peter Bright on Ars Technica:
This smaller release—bug fixes, behind-the-scenes improvements, but little user-visible difference—is likely to be the norm for future Firefox versions. Bigger features will still arrive from time to time, but for the most part, users will just experience a continuous improvement. Firefox updates should be automatic and essentially invisible. Even articles such as this one, which attach some significance to the new release, are probably not what Mozilla wants—press coverage should focus on features, not version numbers. Mozilla—as with Google—wants developers to cease targeting specific browser versions, and instead target standards; the regular releases are one step towards achieving that goal.