Pale Moon x64 will no longer run on Windows XP Pro x64 or Windows Server 2003 x64 from this version onwards!
To continue using Pale Moon on those operating systems, you will have to switch to the 32-bit version. To do this, you will have to uninstall your 64-bit version, and then install the 32-bit version of Pale Moon. Remember to not check to remove your personalizations and data when uninstalling, so your user profile remains intact.
Why is it no longer compatible?
Pale Moon 15.3 uses a new development environment (Visual Studio 2012) which initially had no support for Windows XP at all, but with an update, Windows XP support was added back to it. However, this Windows XP support is on the x86 (32-bit) platform only. Windows XP Pro x64 has never been widely supported, and this backwards compatibility as a result has not been included for the 64-bit compiler and run-time libraries.
Why don't you build a version for us XP-x64/2003-x64 users?
It would mean keeping a separate build environment (Visual Studio 2010), potential compatibility issues in the source code (I had to make changes for building on the new version), and therefore it would be a full separate development branch that I would have to develop and maintain. Some metrics I gathered suggest that only 1.2% of the Pale Moon users are on the affected operating systems, and only a portion of that would actually use a 64-bit browser on that operating system (unknown exactly how much). Combine that with the fact that the 32-bit version runs just as well on the affected operating systems, and it should become clear that it would be disproportionate to do this.
I upgraded my 64-bit browser with the internal updater, and now it won't start!
You unfortunately must not have seen this post before you tried to upgrade. To keep the upgrade process for everyone else as smooth as possible, and because there is no way for the internal updater to check system requirements in this case, you have been served an upgrade package to a version that will not run on your system. Please uninstall your 64-bit version of Pale Moon from Add/Remove programs, and install the 32-bit version from the download page.
What do I lose switching to 32-bit?
Nothing! The 32-bit version built with the new development tools should be very smooth on 64-bit operating systems because of the enhanced features of the new compiler. In addition, you will have more compatibility with plugins and binary add-ons.