How can I tell Notepad++ to always use a particular language with a particular file extension

All you have to do is tell Notepad++ that "xul" files are actually "XML" files.

1. Go to Settings > Style Configurator
2. Under Language, scroll down and highlight "XML"
3. At the bottom, under User ext. type in "xul"
4. Press Save & Close
5. Open your file and it should now highlight properly.

Some other helpful links:

• Assigning custom extensions to a language’s syntax highlighting in Notepad++
• Associating file extensions with highlighters

Here is the instructions for user defined languages. With pictures.

Select: Language --> Define Your Language:

Make sure you omit "." from extension in the Ext. box, or it wont work: