Firefox may be seducing users of Internet Explorer, but Thunderbird, the Mozilla Foundation's free open-source E-mail client, faces less promising prospects against Microsoft Outlook.

A report issued Wednesday by messaging market-research firm Radicati Group Inc. argues Thunderbird won't pull significant market share away from Microsoft, IBM/Lotus, and Novell.

"Thunderbird is not taking off as well as Firefox, but it is getting some interest," says Sara Radicati, the research group's president and CEO. She declined to cite specific usage statistics. The Mozilla Foundation reports Thunderbird has been downloaded 5 million times, though downloads don't necessarily reflect ongoing usage.

"Obviously, Outlook still has a lot of market share and Thunderbird is not going to upset that," she says. Nonetheless, she believes Thunderbird will bring some needed competition to the market. Outlook is the most popular E-mail client in the world, with about 60% of the business market, according to the report.

The report finds Thunderbird offers state-of-the-art security and compares favorably in terms of other features. "If Outlook has a weakness, it's a perception of poor security," the study says, though it notes that Microsoft has made strides in this area and that many users believe there are security issues with any E-mail client.

Thunderbird's major shortcoming is the absence of an integrated calendar, the report says. A future release is expected to fill that void. Once calendaring becomes available, Thunderbird will be much more competitive, the report suggests.

Even so, the study notes that Outlook's position is strengthened by third-party developers. With hundreds of plug-ins available, Outlook users may be reluctant to switch until Thunderbird offers a greater range of expansion options.

Radicati says Thunderbird is likely to appeal to sophisticated users motivated by security concerns, an interest in new technology, or even just the hype. The report concludes that users have nothing to lose by trying it out.