The postMessage() implementation in Firefox 4b8 accepts any value as the first argument to postMessage(), but apparently just converts it to a string and sends the string as the message instead of using the structured clone algorithm as required by HTML5. If you're not going to implement this completely in FF4, I think it would be better for compatibility if FF threw an error if the argument was anything other than a string. Otherwise you run the risk of sites coming to rely on the toString() behavior and failing when the spec is properly implemented. (This bug is probably mis-categorized: I put it in "DOM: Mozilla Extensions" only because that is where the only other postMessage() bug report I could find is.)

postMesage on a worker does the structured clone thing. It's only postMessage on Window objects that ends up doing toString. Ben, is there a reason we don't do structured cloning for postMessage on windows? Or does that just predate the structured clone spec changes?

It just predates. There's probably another bug open.

Ah, indeed.

Sorry for the dup. I'm very surprised that searching Bugzilla for the string "postMessage" does not match "window.postMessage"!

It did for me once I did the search... Might depend on the exact search box you're using, though.