On the following Add-on page, read under "About this Add-on". Always read the Add-on's page and look for "Support" for that Add-on in the right column of the page.

• https://addons.mozilla.org/en-US/firefox/addon/keylogger-220858/

Play Music is the name that the developer of this extension chose to identify this extension within Firefox. Mozilla doesn't allow add-ons to be hidden from view in the add-ons manager tab, so a little bit of subterfuge is used to disguise this extension from "prying eyes".

When I press Alt+L on the browser I am asked to enter a password which I don't have.

Drop an email to the developer asking how to set up the password.

https://addons.mozilla.org/en-US/firefox/addon/keylogger-220858/
About this Add-on
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Important - in any case you need help, please ask us at : lipo.tech[@]yahoo.com

Or do a search of about:config for the preferences related to that add-on.
http://kb.mozillazine.org/About:config

I have dropped the sggested mail and I am waiting for an an answer

The keylogger password seems to be stored in the extensions.pass pref that you can inspect via the about:config page.

• http://kb.mozillazine.org/about:config

Cor-el , thank you for your help. Before I check it I have a question

                              There are two user accounts on my computer : Admin & All , and the 
   Firefox browser functions at the Admin only. Can the keylogger log the presses at the All account

The keylogger is a Firefox extension and as such only works with the Firefox profile where it is installed and enabled.
Note that extensions can easily be enabled by starting Firefox in Safe mode.

See also:

• https://developer.mozilla.org/en/Installing_extensions
• https://developer.mozilla.org/en/Adding_Extensions_using_the_Windows_Registry

You could try a commercial keylogger. That is very easy to use. It is invisible but you can easily see the keylogger as long as you press the hot key.