Disabling Clipboard access of webpages in Firfox

You might also be interested in the ability of websites to see and react to your manual clipboard actions (cut, copy, paste). To block sites from being able to see those actions, you can switch a preference. 

But first, please note... By changing this setting, you gain in privacy and possibly security, but you sacrifice a bit of functionality on some sites that use these features to improve the behavior of some types of editors. It also breaks some features of Firefox's own Scratchpad developer tool. You have to decide whether this is an acceptable trade-off. 

Here's how: 

1. In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful. 
2. In the search box that appears above the list, type or paste clip and pause while the list is filtered 
3. Double-click dom.event.clipboardevents.enabled to flip the value from true (the default) to false.