"Stray doctype" error in firefox source code viewer

I am the developer of this feature. You have found a bug. (Filed just now.) Thanks.

View Source syntax highlighting is based on the HTML parser, because our XML parser is not suited for the purpose and XML is rare enough that it doesn't make sense to put resources into implementing proper XML View Source. Hence, the XML View Source feature is a hack on the HTML parser and this aspect doesn't work quite right.

The error appears because the file is saved as UTF-8 BOM instead of UTF-8. Open the file in Notepad and change its encoding.