One useful newsletter I've been mention for ages is What's New Now. They always pack in a mix of the latest news, critical commentary, articles and tips and tricks in an entertaining and easily digestible format.
I'm currently going unsubscribing from lots of none critical newsletters in a bid to reduce the volume of incoming email, but this definitely one I'm going to stick with. They also have RSS and Podcast versions if you prefer.
The latest edition includes a useful article on Must Have Firefox Extensions.