Firefox 17 will ship with enhanced security that prevents web pages to access data from add-ons. This means that 3rd party websites will not be able to access your data through add-ons, thus creating better security and privacy.
This feature was suggested back in 2010, but will be fully implemented in Firefox 17. Users will have to explicitly whitelist add-ons that they want to be able to access data.
For the time being add-on developers have to make changes to their existing code to ensure that their add-ons will work in future Firefox releases.
The latest Firefox 15 beta release features an error console that helps developers in implementing these changes. Developers will also be able to check their add-ons for compliance with the new policy later when nightly builds of Firefox 17 are out.
Edited by: Zachary Bittner