ThunderBrowse remains one of the best plug-ins for Mozilla Thunderbird. It's a favorite of mine, but if you haven't tried it recently there are some excellent new features worth noting.
If you've never used it before, ThunderBrowse lets you open Web links in Thunderbird's preview pane. It's that simple, which is why it's such an useful tool to have. From social networking friend invites to quickly scanning that hometown article sent by Your Dear Ma, ThunderBrowse cuts out huge chunks of time from your work flow by speedily opening HTML pages in its own browser. The plug-in adds a simple URL bar to the e-mail preview pane, with Forward, Back, Refresh, and Go buttons, and a customizable hot key combo will open the browser in a separate window.