I wanted the new tab to open to yahoo.com (Canadian edition) So I followed your advice and instead of putting in "about:blank" I put in
"about:http://ca.yahoo.com/" - Well that didn't work.
So I looked at what happened and went back and after double clicking on browser.newtab.url, I deleted "about:" so that it looked like "http://ca.yahoo.com/" . Works like a dream.
That is because you used two "protocols". A "protocol" tells the browser /how/ to fetch something. "http" tells it to fetch it from the web, "file" would tell it to fetch a file off your computer. The protocol "about" is used for built-in things like "blank" that don't need to be fetched at all.