Clipboard permissions are browser settings that allow an extension to read copied text from the clipboard, write text to it, or both. In Firefox and other browsers, these permissions are part of the extension trust model: a normal add-on cannot see clipboard contents unless the user grants that access or the browser allows it through a broader privilege.
They matter because the clipboard often carries sensitive material in crypto workflows, such as recovery phrases, private keys, addresses, and login tokens. A malicious or overprivileged extension can watch for copied secrets, replace pasted destination addresses, or exfiltrate data before the user notices. Defenders should treat clipboard access as high risk, review why an extension needs it, and avoid copying wallet secrets into the browser when possible. In practice, clipboard permissions are a common path for browser-side data theft and transaction manipulation.