You can add the following CSS code into your userChrome.css file to hide the search box.

 
@-moz-document url(about:newtab) 
{
  #newtab-search-container, #newtab-search-logo { display:none !important; } 
  } 

You may also want to suppress the large margin at the top.

Note tha the his code should be in userContent.css and not in userChrome.css

/* about:newtab */
@-moz-document url(about:newtab){
#newtab-margin-top {display:none!important}
#newtab-search-container {display:none!important}
}

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

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

• Create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• Paste the code in the userContent.css file

I didn't know about the existence of those archives. I investigated a bit and after a few tests with Firebug I got this code:

@-moz-document url(about:newtab) {

#newtab-margin-undo-container, #newtab-margin-top, #newtab-search-container, #newtab-search-logo {

display:none !important; }

#newtab-grid {

height: 650px !important; max-height: 650px !important; }

.newtab-cell {

height: 9% !important; width: 13% !important; } }

That combines your codes, hides another top margin and resizes the thumbnails in order to fit the 42 squares to my resolution (1280x800). Obviously the values ​​must be modified to be adapted to other number of thumbnails and resolutions. If anyone knows any more generic solution I'll be happy to hear it. I also hope that Mozilla fix this bug to avoid having to do this process. Thanks for your help. :)