Mozilla/5.0 -- basically meaningless any more. It means that the browser is based on Gecko all browsers use it for historical reasons. It means you are using graphical rather than text based browser.
(X11; Ubuntu; Linux i686; rv:39.0) -- The operating you are using, in my case a flavor of Linux
Gecko/20100101 -- the rendering engine I am using, in my case the background renderer for Firefox (Gecko) the date is now meaningless since it became an easy way to track people.
(by the way , in your post above you are using AppleWebKit/537.51.2)
Firefox/39.0 -- the name and the general version of the browser I am using.
Likely more than you want to know on the subject
https://en.wikipedia.org/wiki/User_agen ... tification