From MozillaZine Knowledge Base
This preference’s purpose is a bit vague. It’s been used for plugin-related functions, as well as for determining how tabs and windows should open. It is perhaps best known as a preference to divert URLs from external applications to open in new tabs in an existing browser window. The vagueness is due to the fact that this preference deals with DDE, a method of communication between applications (different instances of Mozilla, or Mozilla and the desktop shell).
In order for Mozilla to use DDE, it must be set to handle the
Possible values and their effects
When started, Mozilla will check to see if it is handling the
This allows Mozilla to act as a DDE server and catch messages sent for the default
Do not set the DDE-related subkeys in the registry.
In Firefox versions prior to 0.9, this preference caused links from external applications to open in an existing Firefox window instead of opening a new window.
First checked in
Has an effect in
Windows versions only (not relevant on other OSes):