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Firefox 3 allows web pages to register themselves as protocol handlers. This allows, for example, all links starting with mailto: to be sent to a webmail provider, or all links starting with webcal: to be sent to an online calendar site. The information on these registrations is kept in mimeTypes.rdf

To prevent websites from registering handlers for other sites and then stealing information meant for the other site, Firefox requires that the host of the URL used by the protocol/content handler matches the host of the site from which the user installs the handler.

Possible values and their effects


Allow sites installing protocol/content handlers to use URLs with different hosts in the handler


Require sites to use hosts matching the current site in protocol/content handlers (Default)


  • This preference controls behavior designed to prevent malicious websites from intercepting data sent with protocol handlers. You should set this to true only if you are aware of the security impliciations.

First checked in

2008-02-08 by Florian Quèze

Has an effect in

  • Firefox (nightly builds since 2008-02-08)

Related bugs

Related preferences

External links