You may remember a time when there were something like 1600 different versions of viewers in Second Life on any given day. The problem with that is it so dilutes crash stats developers are hampered. There isn’t enough data on any single version to provide a developer good information for tracking down a bug.
The Firestorm Team decided they would do something to help their developers. The decision was to limit the number of versions of Firestorm on the grid at any one time to 3. They block their old viewers and prevent users from logging into Second Life with old viewers. I’m not sure if that blocking includes use on other grids.
In this case ‘old’ means not one of the current three newest versions. Currently version 4.4.0 is the fourth running version. The team is announcing that on Thursday they will block that version. There are about 5500 people using that version. They are encouraged to update now rather than wait until they’re blocked on Thursday.