Download the installation package from Mozilla FTP
Firefox version 15 and newer drops support for Mac OS X 10.5 (Leopard).
Just open the disk image and run the installation package. The framework will install itself in the standard /Library/Frameworks path. You may copy the XUL.framwork directory somewhere to be able to install it manually in another machine. If the user does not have the administrator privileges, the framework can be copied locally in ${HOME}/Library/Frameworks/XUL.framework.
For convenience, the framework can be part of your application bundle by copying the XUL.framework directory into Note that the framework is quite large and will greatly increase the whole application bundle size.