Add-ons that can 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 does now verify that the add-ons you install have been signed by Mozilla, digitally. This article explains the add-on signing feature and how it works.
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What is add-on signing?
Mozilla verifies and "signs" add-ons that follow a set of security guidelines. All add-ons hosted on addons.mozilla.org have to go through 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 becoming difficult to track and block the growing number of malicious, or unverified add-ons. The 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 prevents you from installing unsigned add-ons and disables any unsigned add-ons that are already installed.
What types of add-ons need to be signed?
Extensions (add-ons that add features to Firefox) and language packs will need to be signed. Themes (add-ons that change the visual appearance of Firefox) and other types of add-ons do not need to be signed.
Where would I encounter unsigned add-ons?
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.
What can I do if Firefox disables an installed add-on?
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.
What are my options if I want to use an unsigned add-on? (advanced users)
Firefox Extended Support Release (ESR), Firefox Developer Edition and Nightly versions of Firefox will allow you to override the setting to enforce the extension signing requirement, by changing the preference xpinstall.signatures.required to false in the Firefox Configuration Editor (about:config page). To override the language pack signing requirement, you would set the preference extensions.langpacks.signatures.required to false. There are also special unbranded versions of Firefox that allow this override. See the MozillaWiki article, Add-ons/Extension Signing for more information.