Pale Moon SSE is a build of Pale Moon browser optimized for old CPUs with SSE instruction set. Official Pale Moon build is optimized for SSE2, so it won’t work for CPUs like Athlon XP.
Only 32 bit version builds for Windows operating system will be provided.
Since version 25.1.0 the source codes are based on Atom Pale Moon version so that it also supports Windows XP. Developer tools won’t be available anymore.
A full change log is available on the Pale Moon official site.