Chad Perrin of Tech Republic has put together a fantastic how-to for using Firefox's in-built password manager. The article shows you, step-by-step, how to set up a Master Password in Firefox.
Why use a Master Password? Having unique and complicated passwords for all the various websites you use is the most secure method of accessing them. But then you're likely to forget all those passwords. By using the password manager in Firefox, you can store all those passwords, and just remember a single unique password.
This is something you can set up either on Mac or PC following the same instructions, although on the Mac you would access the interface via Firefox > Preferences.
After you set up the password manager, you'll be required to enter the master password whenever you start up Firefox. In order for this security to be useful for you, remember to quit Firefox whenever you leave your computer or whenever you're traveling.
Caveat: using Firefox is not a fool-proof security method for storing your passwords. If you want an even stronger solution, consider using an external password manager such as Password Safe.
While you're at TechRepublic, also check out the recent article about setting IT Security Policies.