Comment 30 for bug 274065

eric, this comment's for you (comment 28 was for kaie):

proxy.js has:
    fileProtocolHandler = CC["@mozilla.org/network/protocol;1?name=file"].createInstance(CI["nsIFileProtocolHandler"]);
which is wrong. protocolhandlers are singletons. the proper way to get one can be found here:
http://bonsai.mozilla.org/cvsblame.cgi?file=mozilla/toolkit/components/downloads/src/nsDownloadManager.cpp&rev=1.53.2.13&mark=1663-1666#1660

mook points out that nsIProtocolHandlers aren't usefully threadsafe, you must get a proxy for them, and in fact, you really want them to give you proxied objects, otherwise what you get is fairly useless.

you should look at some patches i've done involving nsIURIs and crashing (i think i may have even written some of them at your place)