Long time ago, back to 1980′s, X11 define that middle mouse button is paste, you can read the detail in here. As opposed to Microsoft standard uses middle mouse button as navigation key.
Firefox/Swiftweasel in Unix/Linux implemented the same nature of X11. If you have problem with this nature, you can simply disable the feature.
- From your browser, put
about:configin address bar
middlemouse.pasteand change the value to