From MozillaZine Knowledge Base
Jump to navigationJump to search


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 http protocol in Windows and several registry subkeys must be set correctly. This preference determines whether the subkeys are set while Mozilla is running and then removed upon exiting.

Possible values and their effects


When started, Mozilla will check to see if it is handling the http protocol. If it is, it will create and/or set several registry keys:

  • HKEY_CLASSES_ROOT\http\shell\open\ddeexec set to "%1",,-1,0,,,,
  • HKEY_CLASSES_ROOT\http\shell\open\ddeexec\application set to the application name (e.g. Firefox)
  • HKEY_CLASSES_ROOT\http\shell\open\ddeexec\topic set to WWW_OpenURL

This allows Mozilla to act as a DDE server and catch messages sent for the default http-handling application. (Default)


Do not set the DDE-related subkeys in the registry.


  • This preference was originally added to improve compatibility with the Adobe Acrobat Reader plugin (see bug 154238). Setting the preference to false may cause trouble with that plugin under Windows.
  • Using this preference for tab-opening-related functionality is no longer supported. Instead, use browser-supplied UI to set tab behavior (which, in turn, usually sets

Previous effects

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

2002-06-28 by Bill Law

Has an effect in

Windows versions only (not relevant on other OSes):

  • Netscape (all versions since 7.0)
  • Mozilla Suite (all versions since 1.1 Beta)
  • Mozilla Phoenix (all versions)
  • Mozilla Firebird (all versions)
  • Mozilla Firefox (all versions)
  • SeaMonkey (all versions)

Related bugs

Related preferences

External links