Mozilla employee Asa Dotzler celebrated this news in his blog on Sunday.
"That's two million people that won't be getting the next round of Outlook viruses," said Dotzler. "That's two million people who will be able to push the spam aside with Thunderbird's innovative junk-mail filters and get back to using email again rather than being abused by it. That's two million people who will have access to the new and exciting world of RSS."
Firefox, the Mozilla Foundation's browser, has already attracted considerable attention, racking up over 16 million downloads since its release in November 2004.
James Governor, an analyst from Red Monk, said that although Thunderbird will benefit from the attention around Firefox, both still have a long way to go before they achieve mass market status.
"I do think it [Thunderbird] will benefit from this groundswell around Firefox, but we shouldn't get too carried away by that groundswell," said Governor. "For all the momentum Firefox has enjoyed, it still only has a small, technically savvy, user base."
He claimed that Firefox's success among technical users has not yet translated to the corporate space.
"If you look at a lot of blogs, 50 to 60 percent of visitors are using Firefox, but this is not a broad-based enterprise phenomenon," said Governor. "I haven't spoken to any enterprises who have rolled it out end to end."