https://support.mozilla.com/en-US/kb/Remembering%20passwords

Firefox 4 places a key icon on the location bar that allows to (re)open the dialog to save the password at any time.<br /> That pop-up dialog has a button labeled with "Remember Password".<br /> If you click the arrow next to that button then you see the other choices ("Never Remember Password for this site", "Not Now"). Press F10 or press and hold the Alt key down to bring up the "Menu Bar" temporarily. ---- Make sure that you not run Firefox in (permanent) [[Private Browsing]] mode. * In Private Browsing mode some menu items and features are disabled and show as grayed choices. * You are in Private Browsing mode if you see "Tools > Stop Private Browsing", possibly grayed. * You enter Private Browsing mode if you select: Tools > Options > Privacy > History: Firefox will: "Never Remember History" * To see all History and Cookie settings, choose: Tools > Options > Privacy, choose the setting <b>Firefox will: Use custom settings for history</b> * Uncheck: [ ] "Permanent Private Browsing mode" This issue can also be caused by an extension.<br /> See: * https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes