One of the better Mozilla Thunderbird extensions is ThunderBrowse, which allows users to quickly open e-mailed links in a browser window built into the e-mail client itself. The latest version introduces support for the Thunderbird-derived Postbox, as well as compatibility fixes for the Thunderbird 3 beta builds and a new click engine.
However, for users who haven't checked out or updated ThunderBrowse in awhile, there's a lot to play with. Through the expansive options menu, you can configure links to open in new tabs. This allows your original e-mail to stay open in one tab, while the link has been opened in a new one. Unfortunately, in the Thunderbird 3 betas, this does not open a new Thunderbird tab--you'll only get a new ThunderBrowse tab in your message pane. Even if the pane is maximized or the message has been open in a new tab, you'll find a slightly cumbersome new tab bar opening beneath the URL bar.