In order to create an extension, you have to generate GUID for it. GUIDs are also used for identifying other entities, including Interfaces and XPCOM Components.
Windows users can use the GuidGen tool from Microsoft to obtain a GUID. (This tool is also part of MS Visual C++)
/usr/bin/uuidgen. This can be found in package
jkeiser's Mozilla tools include a UUID generator with output format of both C++ and IDL style.
You can get GUID at the page of Shailesh N. Humbad Generate GUID Online. Another generator can be found at the Homepage of family Kruithof - UUID (GUID) Generator on the WEB. Finally, you can usually get a GUID from one of the bots (such as botbot) on #mozilla.