* TB is named after an infectious disease. This means switching program is more effort as you have to learn a new interface. In mail programs however, the program provides the interface. * Browsers have a lightweight interface - most of the interface is in the web page itself. On the other hand, Mozilla's mail/TB lack several Outlook Express features. For example, Mozilla/FB lacks very few IE features that anyone cares about.
* TB is not as universally good a mail client as Mozilla is a universally good browser. If you switch to another mail program on your own (rather than everyone switching), you'll lose those features. * Company users may have extra features (such as integrated calendar and group scheduling) that they require in their corporate email system, such as Outlook. Using a 0.3 web browser is a lot less risky than using a 0.3 mail program. If a web page doesn't load who cares? If a mail program trashes my mailstore I really, really do care. Here are some reasons why people aren't as willing to switch to TB as they are FB: If it was a case of switching from Mozilla mail to Thunderbird then more people would switch, but generally people aren't using either. I think most Mozilla users don't use Mozilla for mail, but use another program such as Outlook or Outlook Express (or indeed those webmail options). I don't think anywhere near that many people use webmail, especially more technically-advanced folk who tend to be Mozilla users.