.Enter about:config in the address bar and hit enter
.You will see a warning. Click the “I’ll be careful, I promise!” button.
.In the filter box, type the following text- "clipboard.autocopy" Change the value of that preference from “False” to “True” by double-clicking or right clicking on it.
.Look for a value named “middlemouse.paste” in the filter bar
.Double click on the value to change it to true.
.Thats it, now you can autocopy by just selecting text and paste when you click the middle mouse button in firefox.