In an era when applications are moving into the web browser, the maker of the world's most popular open-source e-mail client wants you to stay on the desktop.

Later this month, Mozilla will release Thunderbird 2, the latest version of its cross-platform e-mail application. The current version, 1.5, has almost 50 million users worldwide and has been translated into 35 languages. Built on the same technology as Mozilla's Firefox browser, it is loved by many for its advanced filtering features and junk-mail-battling tools, an integrated RSS news reader and the ability to customize with tons of add-ons.

But with popular web-based e-mail services from Google, Microsoft and Yahoo, which just announced that it will offer unlimited storage, the need for a desktop e-mail client seems to be fading.

So we asked Scott MacGregor, Thunderbird's lead engineer, why anyone needs Thunderbird these days, and he had a pretty good answer. He also talked about Mozilla's open-source development model and told us what new features to expect when Thunderbird 2 becomes available.

Wired News: With seemingly every aspect of our data moving toward online apps and away from the traditional desktop model, why is Mozilla still interested in a desktop e-mail client?

Scott MacGregor: We believe the Thunderbird experience is better for moderate to heavy e-mail use. It's much easier to process incoming mail – anyone who's had to use web mail on vacation to deal with dozens of e-mails can testify to how tedious it can be.

WN: What advantages does Thunderbird offer that a web-based app like Gmail doesn't?

MacGregor: Some users want to have their data local for privacy and control. Furthermore, you can integrate data from different applications on the desktop in ways that you can't do with web-based solutions, unless you stick to web solutions from a single provider. For example, you can use your Outlook address book with Thunderbird. We'd like to continue to expand the kinds of data you can share between Thunderbird and other apps (both web and desktop applications).

WN: Speaking of which, Thunderbird 2 has some new integrated web-mail functionality. How does that work?

MacGregor: A lot of users want to check their web-mail accounts using a desktop client, but they don't know all the information necessary to connect. For instance, with Gmail you need to know the server names in addition to your login information. We wanted to make the process easier for users, so we've provided Gmail integration using just an e-mail address. All the user has to do is enter their e-mail address and password and Thunderbird will figure out the server details for them.

For the Mac version, we've provided the same one-step integration with .Mac. We plan to add more web-mail services and even ISP providers in future releases. In the meantime, it's possible for developers to write extensions for other popular web e-mail providers.

WN: Mozilla touts the "open-source security" model as one of Thunderbird's strengths. Why is open-source security better than a proprietary solution?

MacGregor: One of the great things about open source is that you have the entire community, thousands of users, looking to find flaws and vulnerabilities in Thunderbird. And when they do, we have what I call the security SWAT team – people who are always watching for reports of vulnerabilities and helping to patch them. The open-source model allows us to find problems faster, correct them faster and get updates out to users.

WN: What are some of the key features in the new version of Thunderbird?

MacGregor: People still get too much e-mail to easily sort, so we've focused on ways to better manage your inbox. The new mail alerts feature makes it easy to see new mail without having to stop what you're doing and change applications. (Editor's note: This feature is only available in Windows and Linux versions of Thunderbird.)

We've built a number of security features into Thunderbird, like phishing protection. When you view a message, Thunderbird analyzes all the URLs in it to see if they might be trying to trick the user. Thunderbird displays a warning at the top of the message when it thinks something malicious is going on. If the user clicks the links anyway, Thunderbird then pops up a warning dialog. If you're using Firefox as your browser, you really have two lines of defense since Firefox also has built in phishing protection.

We've also added full support for Windows Vista.

We're excited about the creative extensions our community is going to be able to develop with Thunderbird 2. We've really seen an explosive growth of people and companies building add-ons for Thunderbird, so we've tried to make it easier for those developers by improving the add-on framework.

WN: Have many developers updated their add-ons at this point or should the average user wait a little while?

MacGregor: It will take a while for Thunderbird 2.0 to propagate out. Most of the add-on developers wait until RC1 (the first release candidate, when the application has been fully beta tested), when they know the code won't be changing much and then they update their add-ons.

WN: Mac users have been clamoring for support for the Address Book app in OS X. Is that part of the Thunderbird road map?

MacGregor: We're very close to having integration support for Address Book and that will be part of the next release.

WN: What are Thunderbird's plans for the future?

MacGregor: We'll start collecting feedback from users once the new version is released and then we'll sit down as a community and figure out what we're going to do.

We get most of our feature suggestions from user feedback. The popularity of tagging on popular websites like Flickr led people to overwhelmingly want to apply that to e-mail. So tagging support is one of the key new features.