What you might worry about: If you use the same password for every site that needs a password, and one of the sites is unsafe, someone might be able to get your password for that site, and if your name is identifiable, they might be able to try your username + password for other sites. That is nothing the browser can protect you from.
For example, if I could somehow find your MacRumors password, would I be able to buy at amazon using your name? Would I be able to get into your bank account with that name?