XULRunner SDKs can now be downloaded from https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/ Look for filenames like this: Linux 32-bit firefox-.en-US.linux-i686.sdk.tar.bz2 Linux 64-bit firefox-.en-US.linux-x86_64.sdk.tar.bz2 Mac 32-bit firefox-.en-US.mac-i386.sdk.tar.bz2 Mac 64-bit firefox-.en-US.mac-x86_64.sdk.tar.bz2 Windows 32-bit firefox-.en-US.win32.sdk.zip Windows 64-bit firefox-.en-US.win64.sdk.zip where is the current Firefox version. We no longer produce runtimes. Further information see 'XULRunner future and ownership' at https://groups.google.com/d/msg/mozilla.dev.platform/_rFMunG2Bgw/C-4PcHj9IgAJ