It won't be halting development completely, and if you're a current Thunderbird user, you don't have to worry about it going away completely (at least not yet). Mozilla has just decided that Thunderbird, in many cases, already contains most of the features that people want, and it's really only in need of regular maintenance.
Since it's open source, the community can still innovate and develop new features, but Mozilla won't be focusing on it going forward. We're pretty bummed, but at the same time, most of us at Lifehacker have moved to web pages like Gmail as our primary email source, and only use clients like Thunderbird for the occasional backup, or when Gmail goes down. Still, it's a sad moment for those of us that have been using Thunderbird for years. Hit the link to read more about Mozilla's decision.
Thunderbird: Stability and Community Innovation [Mitchell's Blog]