If Thunderbird is specified as your system's default email application, it will integrate with other applications (such as word processors and web browsers). For example, when you click an email address link (like "email@example.com") on a web page or invoke a mail function (such as "Send to") from a word processor, Thunderbird will launch and open a message composition window.
Unfortunately, there is a great deal of variability among operating systems and applications regarding how the default mail client is specified. If the suggestions on this page do not solve your problem, consult the specific application or operating system's documentation for instructions on how to set the default email client. Also, see the Default mail client article on the MozillaZine website for more tips.
Thunderbird can be configured to check if it is the operating system's default mail client each time it is started.
Various versions of Windows have various ways of configuring the default email application. In general, go to Start > Control Panel > Add or Remove Programs > Set Program Access or Defaults. In the Custom settings, configure Thunderbird as the default email application.
The instructions above for making Thunderbird check if it is the default email client should have configured the operating system preference. However, if it isn't working, you can set it manually as follows:
Using Gnome as an example, go to Applications > Desktop Preferences > Advanced > Preferred Applications. Select Custom Mail Reader and enter
thunderbird %s (If Thunderbird is not in the system's PATH statement, specify the full path.)
In some cases (such as Firefox on KDE Linux), the application doesn't check the operating system preferences to determine the default email application. When that is the case, you must set the default in the application itself. Refer to the application documentation for instructions.