Installed Firefox 3.5 Release today and was immediately annoyed at the extra "New Tab" or "Create Tab" button tab that they added to the tab bar. No config option anywhere to turn it off. If you'd like to manually make it go away, you can do so in the userChrome.css file in your Profile directory. You can Google away for Mozilla Profile information and for userChrome.css files if you like. Basically, find your profile directory, add a directory called "chrome" if one doesn't exist and create or edit the userChrome.css file. Insider of it, make sure you have the following:
.tabs-newtab-button {
display: none;
}
Voila, no more extra tab to create new tabs in Firefox 3.5. Hopefully they add a preference for that, or make it available at least through about:config if no were else.