Some websites can benefit from having access to the copy/paste functionality of your system's clipboard. Examples include:
This article describes how to allow certain websites to access your system's clipboard and describes security and privacy issues this could cause.
Therefore, you should only permit the websites that you absolutely trust to be able to access the clipboard.
There is no such functionality built in to Firefox. In addition, there may be third-party Firefox extensions that can help you. For more information on searching for and installing extensions, see Find and install add-ons to add features to Firefox.