Sometimes web page Javascript will disallow you to auto fill your saved password credentials.  In recent versions of Firefox there’s now a way to fill out credentials without digging into preferences -> security -> saved passwords.  Simply enter the username and hit tab.  From there you can right click into the password field and there should be an on option to “fill password”.  Select the user and off you go.  See below for illustration.

Enter Username and hit Tab

Right Click -> Fill Password

Off you Go

Auto-Fill All Credentials
In addition to passwords, Firefox also added functionality completely fill in both username and passwords if they were saved previously.  Right click and use the fill login option.

More Mozilla Firefox password management tips and info can be found here.
Thanks to Kambiz Aghaiepour for pointing out this simple yet useful tip.