How to enable Firefox WebExtensions on Mozilla websites

One of the limitations of the new WebExtensions add-on system of the Firefox web browser was up until now that these extensions would not work on some websites operated by Mozilla.

Only WebExtensions are hit by the limitation, and it means that functionality that these extensions provide is not available on sites and services such as addons.mozilla.org or www.mozilla.org.

Extensions cannot manipulate these pages in any way, for instance to provide additional functionality on them, modify content, or even block elements on them.

Mozilla added a new -- hidden -- preference to Firefox 57 that you can set to do away with the limitation in the Firefox web browser.

Hidden preferences are those that are not visible by default when you open the about:config page of the web browser. They do exist, but you do need to know them to set them in the browser.

The hidden Firefox preference privacy.resistFingerprinting.block_mozAddonManager removes the WebExtensions restriction.

Here is how you set it:

1. Load about:config in the Firefox web browser.
2. You can run a search for privacy.resistFingerprinting.block_mozAddonManager just to make sure it does not exist. It does not in the most recent Firefox Nightly builds at the time of writing.
3. Right-click in the part of the window that lists the preferences, and select New > Boolean from the context menu.
4. Name the new Boolean value privacy.resistFingerprinting.block_mozAddonManager.
5. Set its value to true.

A value of true means that WebExtensions work normally on Mozilla's website. You can undo the change at any time by setting the preference to false on the about:config page of the browser.

Mozilla added the new hidden preference to Firefox 57. It is not available in Firefox 56, and there is no option to introduce it in the browser. Firefox Stable users who run WebExtensions right now need to wait until the browser is updated to version 57 to set the preference and gain the functionality