From slashdot user  ciaran2014 (3815793) 2015-08-14 19:51

Summary: 
   Firefox's own How to stop Firefox from making automatic  connections  also
   mentions  setting  network.http.speculative-parallel-limit to 0 to to stop
   predictive connections when a user "hovers their mouse over thumbnails  on
   the  New  Tab  Page or the user starts to search in the Search Bar" but no
   mention regarding  hovering  over  a  normal  link.   Good  thing  setting
   network.http.speculative-parallel-limit   to  0  does  appear  to  disable
   speculative connect on normal links too.  One can expect Firefox  to  make
   requests  in the background to its own servers for things such as checking
   for updates to plugins etc.  But silently making requests to random  links
   on  a  page (and connecting to those servers) simply by hovering over them
   is something very different.

And for anyone new to Firefox, to set that variable:

1. Type "about:config" into the address bar (and you'll see a list of
variables)

2.   Copy'n'paste  "network.http.speculative-parallel-limit" into the
search bar at the top of that page and hit Return.

3.  You'll now just have that one line on the page.  Double-click  it
(or right click on it and select "Modify").

4.  A box pops up, you change the value to 0, and hit OK.

Done.

(The  first  time  you  look at "about:config", Firefox might ask you
"Are you sure you know what you're doing?" Obviously you say  yes  to
this.)

(Yes  I  know  I've explained it as if talking to a ten year old, but
protecting  your  privacy  is  important  so  it's   important   that
absolutely everyone can do it.)