I just tried removing and then adding items from my reading list, and hit this: Sending message that cannot be cloned. Are you trying to send an XPCOM object? ReadingList.jsm:430:0 (https://hg.mozilla.org/mozilla-central/annotate/883e17fc475f/browser/components/readinglist/ReadingList.jsm#l430) Sending message that cannot be cloned. Are you trying to send an XPCOM object? ReadingList.jsm:349:0 (https://hg.mozilla.org/mozilla-central/annotate/883e17fc475f/browser/components/readinglist/ReadingList.jsm#l349) I don't see any sync logging so this seems to break syncing. Drew, any ideas here?

Hmm, bug 1139718 ran into something like this? What's in the |item| being sent?

Bug 1149896 is on file for this, but I'm not sure what we're going to do about it. I don't think it should break sync though, but it might break the front-end's handling of synced items, which may end up looking like sync is broken. You may have to set one of readinglist.log.appender.dump or readinglist.log.appender.console to "Debug" to see logs show up in the terminal or browser console, respectively.