You can remove your computer from sync by following these steps:

Click the menu button to expand the menu. Click on your Sync account name (usually your email address) to open the Sync preferences. Click Disconnect. Your device will no longer be synchronized.

I'm not trying to remove my device from sync. Just for some reason when I open firefox,it ask me to sign in again. When I do, I get an email saying a new device is now syncing. I'm trying to figure out how to stop getting the notification. It's not a new device,it's the same device just had to log in again. Sorry my original question wasn't clear.