Firefox by default does not use the master password option, before you can use it you have to define it.
Step 1: Open your Firefox web browser and click on Firefox menu at the top left corner, navigate to options and then click on options.
Step 2: Now move on to security tab in options window
Step 3: Next in there you will see an option called “use a master password”. Just check that and enter your master password twice. Also you can see the password quality meter, to protect the information make sure that you have set strong password.
Step 4: When done click on OK and you will see a message that says “master password successfully changed”, click on ok again. That’s it!
Now clear your browser cache
and now open any site that Firefox has previously stored your usernames and passwords. Before it fills your login details on that particular site you will see a message like this where you have to enter your master password.
From now on nobody can use or view your usernames and passwords in Firefox unless they know your master password. Also you can remove or change your master password in feature by going to options >> security tab but to do so you have enter the current password.
If you like to add some additional features to the built in master password you can try the master password add-on
for Firefox. After you install this add-on you will be asked to restart your web browser. When done you will see a small lock icon at your address bar, just click on that and you will get more features like auto logout, auto lock etc.