GUIDs are used in Mozilla programming for identifying several types of entities, including XPCOM Interfaces, components, and addons, like extensions and themes, although addons can also be identified with IDs of form <tt>firstname.lastname@example.org</tt> since Firefox 1.5.
You can use a number of tools listed here to generate GUIDs.
You can create a 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 IRC channel.
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.
A UUID can be generated from privileged Mozilla code using
nsIUUIDGenerator. See the linked page for details.