This topic has been given a lot of thought lately, and considering the fairly small performance difference between SSE and SSE2 on the 3.6.x browsers, the double release of 3.6.x will be discontinued from here on out. This is also fueled by the better performance of Pale Moon 6.x and the upcoming 7.x version on Windows XP (the v4 and v5 kinks are being worked out for that platform in the current versions)
This does not mean that development on v3.6.x stops!It means that the 3.6 builds from here on out will be SSE builds, considering the average systems they are run on (hardware/OS combination). This makes things simpler for the end user, takes less time and energy to maintain, allows for better focus on fewer different versions and will, overall, serve to provide a clearer release framework for everyone involved.
Starting with 3.6.23, there will be a unified version for all supported systems (Athlon XP and up) of this branch.
Next generation (v4 and later) builds, of course, will continue to use the full SSE2 instruction set as they have been.