Are you referring to the nine thumbnails that appear on a new tab, in place of the earlier blank page? You can return to the blank page with a change in the about:config preferences editor. Please try the steps in this article: New Tab Page – show, hide and customize top sites > How do I turn the new tab page off?. Any luck?

Edit: Moved as appropriate.

No, I am referring to App Tabs, not the frequently visited sites found in the new-new tabs page. Sorry!

Edit: Seems I can't reply to the reply -- only keep adding on. I learned something new today!

You can probably use the Menu Editor extension to remove the Pin as App Tab entry in the right-click context menu of the tab bar.

• Menu Editor: https://addons.mozilla.org/firefox/addon/menu-editor/

You can also add code to the userChrome.css file below the default @namespace line to hide the context menu items.

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

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

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

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

#context_pinTab, #context_unpinTab { display:none!important; }