User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1a2pre) Gecko/20080817040141 Minefield/3.1a2pre Build Identifier: When URLs are clicked on in Thunderbird, the URL is sent to the browser. This works alright in OS X, when most people leave their browsers running, but on Windows, people probably keep that task bar pretty clear. This is a suggestion to add an option or make it default for Thunderbird to render URLs in a new Thunderbird tab. This offers a couple of advantages: 1) A new browser instance doesn't have to open if needed to visit a webpage. I would think most people don't continue using that new tab to explore other parts of that site or the web beyond another page or two, so it's a relatively short-lived tab. This means we need to wait to launch a new browser to explore a site or article for under a minute. 2) UI control stays with Thunderbird meaning that switching back to the message you were reading will be a CTRL+W shortcut away. This makes it easy to read an external reference quickly and then return to reading your original email/RSS item (external links are very common in blog posts and a lot of time are just given as a reference or background information). Reproducible: Always Steps to Reproduce: 1. 2. 3. Related to 222021, especially last comment.
Thunderbird isn't a web browser, so I'm not sure what you're asking. Firefox has settings that let you open a new tab rather than a new window. I haven't played with SeaMonkey (the combined browser/E-Mail suite), but it sounds like that might be more the route you're looking for.
Of course its primary function isn't web browsing, but it is built on top of all the web technologies Firefox uses and can render similarly. I'm not looking for anything but Thunderbird, this is merely a feature request.
We will not implement this functionality in Thunderbird core, but we are working hard to make sure that it is easy for extensions to provide support for web browsing (either site-specific or more general).