The concept of presence in
unified communications aims at putting your contacts at reach, wherever you might be. It is very convenient to just click on a contact's name or any personal information about them in their email messages or contact details to call them, without actually requiring to launch a
softphone. And the call can be free. This can be achieved by integrating a VoIP softphone service like
Skype into your
Thunderbird email client. Here are ways you can achieve that.
For Mozilla products like Firefox and Thunderbird, I look for plugins before anything else, as these are normally free and better adapted to the software. The Skype toolbar plugin is free, and has basic functionality for initiating Skype calls, chats and conferences in Thunderbird.
This piece of software does the basic job. I have found more complaints for this product than for the plugin above. It is free.
This little plugin, developed by seemingly a hobbyist, gives a context-sensitive menu on right-click, in which there are the main Skype functionalities. There exist versions for both Windows and Linux. It is free.