WebExtensions is the browser add-on framework used by Firefox and other modern browsers. It lets an extension declare permissions and run scripts, including content scripts that can inspect or modify web pages inside tabs. With the right permissions, an add-on may also interact with clipboard data, page forms, and site content.
This matters because browser extensions sit in a trusted position between the user and the web. If a malicious or overprivileged extension is installed, it can become a collection point for passwords, session data, recovery phrases, or private keys entered into browser-based workflows. Defenders reduce risk by reviewing requested permissions, limiting extension installs, using add-on signing and policy controls, and avoiding pasting high-value secrets into the browser when possible. In real attacks, WebExtensions abuse often looks like a normal utility add-on that quietly reads page data or captures clipboard content.