Add-ons that change your browser's settings without your consent or steal your information have become increasingly common. Some add-ons can add unwanted toolbars or buttons, change your search settings or inject ads into your computer. Firefox will now verify that the add-ons you install have been digitally signed by Mozilla. This article explains the add-on signing feature and how it works.
Mozilla verifies and "signs" add-ons that follow a set of security guidelines. All add-ons hosted on addons.mozilla.org undergo this process in order to be signed. Add-ons hosted on other sites will need to follow the same guidelines in order to be signed by Mozilla.
While Firefox currently has a blocklist system, it is increasingly difficult to track and block the growing number of malicious add-ons. The new add-on signing process requires developers to follow Mozilla Developer guidelines. Add-on signing in Firefox helps protect against browser hijackers and other malware by making it harder for them to be installed. Firefox will warn you about third-party add-ons that are not digitally signed by Mozilla. For now you can still install the unverified add-on at your own risk.
In Firefox version 43 and above, Firefox prevents you from installing unsigned add-ons and disables any unsigned add-ons that are already installed.
Extensions (add-ons that add features to Firefox) will need to be signed. Themes, language packs and plugins do not need to be signed.
Add-ons installed through the official Firefox Add-ons site go through security checks before they are published. These add-ons are verified and signed. When you install an add-on through another website, Firefox checks to make sure that the add-on is digitally signed.
If an unsigned add-on is disabled, you won't be able to use it and the Add-ons manager will show a message that the add-on could not be verified for use in Firefox and has been disabled. You can remove the add-on from Firefox and then reinstall a signed version from the Mozilla Add-ons site if one is available.
If a signed version is not available, contact the add-on developer or vendor to see if they can offer an updated and signed version of that add-on. You can also ask them to get their add-on signed.
You can override the setting to enforce the add-on signing requirement by changing the preference xpinstall.signatures.required to false in the Firefox Configuration Editor (about:config page). Support is not available for any changes made with the Configuration Editor so please do this at your own risk. Starting with Firefox version 48, the add-on signing requirement will be enforced with no override in Release and Beta versions of Firefox. See the MozillaWiki article, Add-ons/Extension Signing for more information.
Firefox ESR version 45, as well as the Developer Edition and Nightly versions of Firefox, will allow you to override the setting to enforce the add-on signing requirement by changing the preference xpinstall.signatures.required to false in the Firefox Configuration Editor (about:config page). There are also special unbranded versions of Firefox that allow this override. See the MozillaWiki article, Add-ons/Extension Signing for more information.