That's weird. Could you provide some sites URL as example, or it's completely random?
The button does not show most of the time if you select text on the web, and when the button does appear, clicking on it does not show anything, am I right?
Remember that "green button" does not support all sites, like this one (addons.mozilla.org).
And what are your firefox version number? If you have time to provide more detailed information, you can contact with me by my email tranthang.yb@outlook.com.