Perhaps the introduction of Javascript Code Modules will make Javascript-to-Javascript XPCOM communication almost unnecessary. I know I was very thankful of being able to get rid of XPCOM when working on the most recent extension I co-authored (Fire.fm).

(From Alex: Don't bet on it. The trend these days is for more components to be written in JS, not less - and XPCOM does offer strong types, which JS modules don't.)