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This article covers security-related changes in Firefox 3.5.
Changes to chrome registration
A security hole was closed in order to prevent remote content to be used as chrome. This could impact any add-on that included a resource in their chrome.manifest file that referenced a file on the web.
Fixing this bug was accomplished by adding a new URI_IS_LOCAL_RESOURCE flag to the nsIProtocolHandler interface that indicates that the protocol is safe to register as chrome. Any add-on that creates its own protocol handler and tries to register it in its chrome.manifest file will have to use this flag in order to work correctly.
Private browsing
Firefox 3.5 implements private browsing, a mode in which cookies, history, and other potentially private information isn't saved permanently on the user's computer. Extensions and other add-ons may support the private browsing feature, detecting when it's in use so they can avoid saving private information while private browsing mode is enabled. See Supporting private browsing mode for details.
Plug-ins can detect whether or not private browsing mode is in effect by using the NPN_GetValue() function to check the current value of the NPNVprivateModeBool variable.
New certificate error handling
In previous versions of Firefox 3, SSL certificate errors resulted in the presentation of the standard network error page, about:neterror, in the browser window. Starting in Firefox 3.5, there is a new error page, about:certerror, which is displayed instead. The error URL is formatted like this:
about:certerror?e=error&u=url&d=desc
Embedders needing to provide custom certificate error pages can now do so by supplying their own about: page implementation, and setting the security.alternate_certificate_error_page preference to the appropriate page name (e.g. "certerror").