You should be able to do this with code in the userChrome.css file.

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#newtab-window a.newtab-link[href*="facebook.com"] span.newtab-thumbnail{
 background-image: url("<path to image>")!important;
}

The customization files userChrome.css (interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

Sorry, I'm new to this.

I'm using ubuntu (12.04). I tried to search for the file

- find ~/ -type f -name "userChrome.css" - locate userChrome.css

Without any results. Where do I find the "userChrome.css" file?

Thanks in advance. Regards.

The userChrome.css file doesn't exist by default.
You need to create a chrome folder in the Firefox profile folder and a plain text file called userChrome.css in that chrome folder.

This extension will create the 3 "user" files for you, and gives you an interface within Firefox for editing those files.
http://webdesigns.ms11.net/chromeditp.html

I installed it. But what do I have to do to create the 3 user files? I checked the checkbox where it says the "Open ChromeEdit in a Tab", but when I opened a tab there was no ChromeEdit. When I clicked on the "Open the Chrome folder"-Button, a windows opened and asked me to choose an application to open the chrome folder.

Could you please help me to modify firefox? Again, I'm new here .....and a little bit clueless here.......-.-

Did you install the extension?

In that case you need to check the Tools menu or possibly its Web Developer sub menu to open ChromEdit.
Go to the userChrome.css tab and paste the code in the edit window.

Easiest is to place the images that you want to use in the same chrome folder where the userChrome.css resides because then you only need the file name and no path, but an absolute path or internet URL should work as well.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Open Directory