Bug 582712 killed the only use of it, AFAICT, and bug 700314 removed the unused implementation in Firefox. I don't see any hits for its methods in comm-central or the addons MXR. We should kill it!
Created attachment 581585 [details] [diff] [review] Patch v1 https://tbpl.mozilla.org/?tree=Try&rev=7ce27fb4cc9d
Interface marked as obsolete, noted on Firefox 11 for developers.