This is from my personal experience: It almost happens every time I tried to connect with Sync from my Android phone, the window asking me to enter my account name and password always shows up. And if I enter the information, it'll say I have the wrong password, etc. My workaround is to click the "cancel" button. But it does not cancel immediately. The window will stay on for a while (usually a few minutes), then it'll disappear and start connecting to Sync. And it'll connect and sync successfully. You can try this workaround but you have to be patient to wait for the window to disappear. I don't know what is the cause of this, but it has happened ever since FF4 for Mobile came out. I have searched around and couldn't find any solutions.