569 bytes, text/html
Pretty sure there's an existing bug about the origin tracking on slow script being broken (e.g. through event handlers, etc).
I searched - maybe it has some non-obvious title.
For better dupe-matching, the RSnake demo described above is at http://ha.ckers.org/weird/side-dos.html
bz, regarding comment 1 (dupe) are you thinking of bug 78089 "A script on this page is causing mozilla to run slowly" message is too vague and badly worded or Bug 397394 - unresponsive script should identify culpable tab and bug 573310 states the script itself isn't running long, but the firefox stuff it calls is running too long? I didn't find anything else. although bug 470765 about improving diag, but is is mostly WFM