Add-on signing is Firefox’s control for verifying browser extensions before they run in release builds. A signed add-on has been reviewed and approved for distribution, which helps prevent unreviewed, tampered, or unsigned extensions from being installed in normal user environments.
This matters because extensions can read page content, inject scripts, and sometimes access clipboard or other sensitive browser data. In attacks, a malicious add-on may look legitimate while collecting passwords, recovery phrases, or other secrets from inside the browser. In defense, add-on signing helps reduce the chance that a rogue extension reaches users through the standard release channel, but it is not a complete safety guarantee. Security teams still need permission review, extension inventory, and user awareness, especially when a browser add-on requests broad access to wallet, trading, or productivity sites.