Copy Guard от ruudschroen

This extension warns you whenever your copy action was hijacked. When you select text and copy it, this extension will compare your selection with the data that was added to your clipboard. If there is a difference, a native notification will be triggered.

Since version 1.1.0 you will receive a warning if your text selection contains hidden or offscreen elements.

IMPORTANT NOTICE: This extension asks for permission to "access your data for all websites". While this may seem invasive, it is required for injecting a script upon loading the page. This script will listen to the copy event and retrieve your current text selection. Please rest assured that no other data is being collected. The source code of this extension has been reviewed by Mozilla and is available on Github.