Do we need email software anymore? That's the question raised by the turmoil at the Thunderbird project, an open-source effort run by the Mozilla Foundation, also the backers of the popular Firefox Web browser. The foundation runs the Mozilla Corporation as a separate, for-profit business, and spun off a similar company three weeks ago to house Thunderbird. But no sooner than Thunderbird gained its wings did it go into a swan dive. Scott McGregor and David Bienvenu, Thunderbird's only paid developers, are leaving David Ascher to head up a company of none. Neither developer gave a reason for departing the company that Mozilla set up, but Ascher tips his hand that the pair will be starting their own venture. Perhaps a wise move. Why, after all, do we need Thunderbird?