Mozilla uses IDL for in two ways
Web IDL: used in W3C specifications and such to describe Javascript APIs. These are the normative specifications.
XPCOM internal IDL dialect: The native Javascript APIs are implemented in C++. In Gecko's (Firefox's engine's) case, specifically in domain specific framework called XPCOM
As the linked page states, Gecko internally describes interfaces in language neutral IDL dialect, because those interface must be implemented both in native run-time (C++) and Javascript engine (Javascript).
In this case the IDL description is either copy-paste from the orignal Web IDL specification or from Gecko's internal implementation.