On August 21st 2015, Mozilla announced they will remove XUL and XPCOM support from add-ons in 12 to 18 months. They should not do this, as thousands of add-ons have been written in XUL benefiting millions of users. If this change is to be made, many add-ons that people use will be impossible to create due to the free and open nature of XUL, and many other add-ons have been abandoned by their developers. Developers of current add-ons will have to rewrite their add-on, and many developers will decide not to, as they may not have the time to do so. Mozilla has decided this because it wants to increase user security and make add-ons easier to port over from Google Chrome by introducing a new API, which I believe is a good idea, but it’s not a reason to remove XUL, these APIs should be able to coexist. One of the many reasons I, and many other users, use Firefox over Chrome is the ability to customize the browser through add-ons and userChrome.css, and removing XUL support is a major threat to this ability. Mozilla cites “However, the add-on model that arose naturally from these technologies is extremely permissive. Add-ons have complete access to Firefox’s internal implementation.” as the reason for considering removing XUL. While this is a valid point, these permissive add-ons are one of the major upsides that Firefox has over its competition. Add-ons can do so many things in Firefox that would be impossible in other browsers, such as complete themes, because of its permissive nature. By removing XUL and XPCOM, you will alienate your developers and users, and some of these users may move to other browsers. This is why I ask you to keep XUL and XPCOM support for add-ons as well as the new add-on API, this way you can have the maximum amount of add-ons, legacy and new.