MDN’s new design is in Beta! A sneak peek:

Mozilla preferences for uber-geeks

There are a lot of "secret" preferences in Mozilla that give you control over specific features or areas of the software that may not be safe for typical users to play with. This article provides a list of some of them.

Warning: If you don't know for sure what you're doing, don't change these preferences! Doing so could cause any number of problems!

Security preferences

Preference name Possible values Notes
browser.xul.error_pages.expert_bad_cert false (default)
By default, the "Add exception" button is not available on the error page that appears when an SSL certificate is invalid. If you set this preference to true, that button is restored. If you also set the browser.ssl_override_behavior preference to 2, the certificate information is prefetched into the error dialog, saving you another click to override the error.

DOM preferences

Preference name Possible values Notes
dom.min_timeout_value Positive integers The minimum length of time, in milliseconds, that the window.setTimeout() function can set a timeout delay for. This defaults to 4 ms (before 10 ms). Calls to setTimeout() with a delay smaller than this will be clamped to this minimum value.
dom.max_chrome_script_run_time Positive integers The maximum number of seconds that scripts running in chrome are allowed to run before being timed out.
dom.send_after_paint_to_content false (default)
For security reasons, sending the MozAfterPaint event to web content is disabled by default for the time being; it is still sent to chrome, however. Setting this preference to true enables the event.

Other preferences

Preference name Possible values Notes

true (default)


If true, the new tab-modal alert style is used; if false, the old window-modal alerts are used instead.

Document Tags and Contributors

 Contributors to this page: fmarier, Aryx, lucasbetschart, Sheppy
 Last updated by: fmarier,