On Mozilla Firefox, type in search dialog box., “stylish”. Click on the link with this result.
Stylish-Add-ons for Firefox.
This screenshot should appear. To be able to download and install the add on click the add to Firefox. Be sure that you are using Firefox as your default browser.
A dialog box will appear. Asking for confirmation to install the file.
Click install to execute the installation of Stylish-addons for firefox. 
A restart will be needed to refresh and complete the installation. 
click restart firefox. 
To confirm that the add-ons is installed click on the tools---> add-ons.
Now you are ready to install your Free Facebook skins.
