The existing MessagePort specification does not work with multiple threads or processes well, and doesn't appear to be easily fixable. Besides, Mozilla is about to ship an entirely different and much simpler API for workers. Patch forthcoming.
Created attachment 25173 [details] proposed patch
Comment on attachment 25173 [details] proposed patch r=me I hope this reaches the point of testability soon so that we can document behavior changes like this with test case changes.
Committed revision 38443.