Firefox 8 will automatically block web browser add-ons that are installed by other software. Users may manually approve the add-ons and make them active, but until they do, the add-ons will be disabled by the browser. Software-bundled add-ons have been a pain for users--many users are troubled when they find software and add-ons on their systems that they never installed.
For example, Microsoft quietly slipped a Skype add-on into Firefox, which left their users open to attack and lead to numerous web browser crashes.
Add-ons can certainly slow down browser performance and render the internet browsing experience less safe for users.