Some forums and possible other services have a profile or login setting to hide the login status, so you need to look for such an option.

About which website is your question?

My brother said he can tell when I come on line. He gets notification of it. It is either with Internet Explorer or MSN or both.

Same house and you're connected to the same router?

Does your brother has the internet connection and is sharing that connection with other computers via the network connection?

No to both.

Can you please specify a little more?

Does this happen when you connect, or just when opening Firefox? Can he tell all the time, or only after you go to a website?

This doesn't sound like something from Firefox. It sounds like your brother may have: 1. A direct connection 2. A notification via Facebook, Gmail, forums, ect. 3. A program similiar to the Find-My-Friends app on an iPhone

You should probably talk to your brother as he may have more information.

Look for any applications that start on startup and you have it set so you will be logged in.