nsIUUIDGenerator
1.0 66 introduced gecko 1.8.1 inherits from: nsisupports last changed in gecko 1.8.1 (firefox 2 / thunderbird 2 / seamonkey 1.1) implemented by @mozilla.org/uuid-generator; as a service: var uuidgenerator = components.classes["@mozilla.org/uuid-generator;1"] .getservice(components.interfaces.nsiuuidgenerator); method overview nsidptr generateuuid(); void generateuuidinplace(in nsnonconstidptr id); native code only!
...nsidptr generateuuid(); parameters none.
... return value this method returns a nsidptr containing a unique id.

XPCOM primitive
(however, if you are designing that kind of api today, you should probably use nsivariant instead.) idl data type interface component idl nsidptr nsisupportsid @mozilla.org/supports-id;1 [scriptable, uuid(d18290a0-4a1c-11d3-9890-006008962422)] interface nsisupportsid : nsisupportsprimitive { attribute nsidptr data; string tostring(); }; astring nsisupportsstring @mozilla.org/supports-string;1 [scriptable, uuid(d79dc970-4a1c-11d3-9890-006008962422)] interface nsisupportsstring : nsisupportsprimitive { attribute astring data; ...
...4f0-568d-11d3-baf8-00805f8a5dd7)] interface nsisupportsvoid : nsisupportsprimitive { [noscript] attribute voidptr data; string tostring(); }; nsisupports nsisupportsinterfacepointer @mozilla.org/supports-interface-pointer;1 [scriptable, uuid(995ea724-1dd1-11b2-9211-c21bdd3e7ed0)] interface nsisupportsinterfacepointer : nsisupportsprimitive { attribute nsisupports data; attribute nsidptr dataiid; string tostring(); }; ...

nsIJSID
method overview boolean equals(in nsijsid other); const nsid* getid(); violates the xpcom interface guidelines void initialize(in string idstring); string tostring(); attributes attribute type description id nsidptr read only.

nsISupportsID
inherits from: nsisupportsprimitive last changed in gecko 1.2 method overview string tostring(); attributes attribute type description data nsidptr provides access to the native type represented by the object.

nsISupportsInterfacePointer
dataiid nsidptr stores an iid corresponding to the data attribute.

XPIDL
nsidptr const nsid* nsid** ?