hello, do you just want to hide the icon or disable the whole feature?

I still struggle with understanding userChrome.css coding, so I don't have a complete answer.
http://kb.mozillazine.org/UserChrome.css

I have used this code in userChrome.css to hide that entire button panel.

 @-moz-document url(about:home){
 #launcher { display: none !important; }
}  

Here's the code for the spacer and that restore last session button from the Page Source of about:home..

<div id="launcher"> 
<div id="restorePreviousSessionSeparator"/>
      <button class="launchButton" id="restorePreviousSession">&historyRestoreLastSession.label;</button>
    </div>

Hopefully someone else will out it all together for you to block that specific button and the spacer to the left it it.

This button is only present on the about:home page, so you can consider to use another page as the home page.
Of course you will still have the "Restore Previous Session" entry in the History menu.

Do you want to remove (hide) it there as well?

See also:

• http://kb.mozillazine.org/Session_Restore
• https://support.mozilla.org/kb/restore-previous-session