In previous versions, setting browser.newtab.url to about:blank disabled the newtab grid.
Now that I've updated to Firefox 18, Firefox doesn't remember the setting.
When I restart Firefox, it changes the browser.newtab.url that I had set to about:blank back to about:newtab.
Go to Solution.
I found an option in Tab Utilities that took care of the problem.
Thanks for your response.
If you do not keep changes after a restart then see: