IE and Firefox both permit you to store web site credentials for automatic re-use. Such a feature makes it more convenient for users to sign on to frequently-used web sites without having to type in a password every time.
I recommend you NOT do this.
In my opinion, use of a browser for storing and managing web site credentials is risky business. A browser is a virtual terminal that is used to access web applications, some of which contain malicious code. Such malicious code has little distance to travel between a browser and its stash of stored passwords.
In fact, already such exploits have been written in what are called “Reverse Cross Site Request” attacks, in which a fake login site fools the browser’s password manager into automatically providing login credentials.
And here is an article that explains these vulnerabilities further:
Instead of using browser-based password managers, I suggest you use a separate encrypted password vault such as Password Safe.