Team Firestorm has a "three version rule," dating from an agreement with Linden Lab years ago. So the team has to block older versions of it's viewer. The grace period for version 188.8.131.52052 ends today, and it will be blocked tomorrow.
The latest update promises bug fixes, and features including a "massive" RLV update, and a "New free-aim mouse and focus lock for Depth of Field" for screenshots.
Source: Firestorm blog