Mozilla's email client may not be as popular as the company's web browser, Firefox, but it is an impressive application that sways many doubters when they eventually try it out. The program can be used to send and receive email from a wide variety of accounts, including Google's Gmail, but by default it is not able to take full advantages of all of Gmail's features.
Thunderbird Conversations is a free add-on for Thunderbird that can be used to add a Gmail-style conversation view to your inbox. This special viewing mode can be used with any type of email account, not just Gmail, and it provides an easy way to keep track of email exchanges by viewing them as threaded conversations rather than individual messages.
Despite the name, there is far more to Thunderbird Conversations than just a new message display mode. The add-on can also be used to view details about your contacts - such as Twitter and Facebook information - by hovering the mouse over their name in an email.
In addition to this, Thunderbird Conversations enables you to quickly reply to emails using a special inline mode that greatly speeds up the process of composing an email, while support for autocomplete and keyboard shortcuts helps to speed things up further. Despite the fact that this is still an early alpha version of the add-on, it works well and appears to be largely stable.
If you are a Thunderbird user, this is well worth checking out, and if you are not yet using the Mozilla email client, this may be the reason you have been looking for.