FF 3.5 remove newTab Button

rename /home/romani/.mozilla/firefox/u3p8un3p.default/chrome/userChrome-example.css
to //home/romani/.mozilla/firefox/u3p8un3p.default/chrome/userChrome.css

add ".tabs-newtab-button { display : none };"
after "@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */"

If no such file "../chrome/userChrome.css" - create it and put:
.tabs-newtab-button { display : none };