Obsolete for FF13 or later Rated 5 out of 5 stars
I've used this add-on for a while with great results, but it has been rendered obsolete in Firefox version 13. Similar to miko007's tip from 4/29/2012 below, you can now duplicate the behavior of this add-on within Firefox itself, and specify any URL new tabs should point to (in FF13 or later).
For example, if you want new tabs to always show Google, first type "about:config" (no quotes) in the URL bar , acknowledge the "I'll be careful" warning, scroll down to the "browser.newtab.url" entry, double click on it and type "http://www.google.com/" (again, no quotes) in the pop-up box. You can type whatever URL you'd like new tabs to go to, doesn't have to be Google. Once you've done this, AFAIK, you can disable or remove this add-on.
Other tips w/ new tabs: to show a blank page, use "about:blank" in the browser.newtab.url entry. To restore the newer thumbnail tab page, use "about:newtab".