The following was copied/pasted from https://forum.palemoon.org/viewtopic.php?t=12337&p=86995#p86995
We currently have a specialized Atom-optimized build to serve both Windows XP and Atom CPU machines specifically as a courtesy to people running those operating systems or hardware.
In preliminary performance testing, it became clear that (likely because of the complexity and size of our code base), Atom-optimized binaries don't perform significantly better than regular builds of our next milestone (v27.0) and as such we won't be continuing this specialized build from the next milestone release forward.
This will make things significantly less complex, both for us and for the user, and allows us to focus on a single path with no special exceptions or workarounds for problems, ultimately leading to a better browser, because we no longer have to maintain multiple branches as we do now.
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Remark: PaleMoon now joins IE(8) and Google Chrome in finally dropping support for XP. That leaves only Firefox. It will be interesting to see how long FF will continue to support XP.
[Opera 37 still offers security and stability updates for XP/Vista, but not feature updates.]