LJ seems to have done something to the challenge/response login mechanism that breaks Post History and LJ Sync. This is quite annoying. I am poking through trying to figure out how to get around it, but for now, if your copy of Firefox hasn't decided to update itself and break all your addons, try going to the account options (on the DS login page, hit the Options button) and unchecking the "use challenge/response authentication" part. This should get things working again for now if you're really desperate, however it's pretty insecure (not as bad as plaintext, but close - it just sends an MD5 hash of the password to the server). A proper update will be on its way... soon.