I can't get the loaded message's URI with GetLoadedMessage(), from a load event handler. I was able to do this until 2006-11-24 nightly; since 2006-11-25 it fails. (not sure this is the right component, the reorg is confusing)
Are you talking about the message window, or, the main three pane messenger window? I found two GetLoadedMessage() functions in SeaMonkey.
It's weird that nobody else has noticed this before.
Well OnLoadMessenger() calls setTimeout(delayedOnLoadMessenger, 0); which then initializes things. Perhaps you could check for |gHaveLoadedMessage| before trying to run GetLoadedMessage()?