Both Firefox and Chrome store passwords in an obscured form but not encrypted. In Firefox you can set a master password which causes the passwords to be encrypted for storage. Of course then you have to enter the master password in order to use the stored passwords, so most people don't bother. I assume that Chrome does something similar.
This means that it's not hard for either browser to use the passwords stored by the other. In fact, it means that any program running on your computer can recover those passwords, so you should exersize some care when choosing programs to install, or set a master password so that the passwords are encrypted.
