Hi irban - ok, I used the "Add to Firefox" but have just tried this way (I assume it's what you've done) -
"Click here to install the latest v4.1"
"Install"
"Restart Now"
That reopens their home page on a "Tab" - pic here shows tabs -
http://media5002.dropshots.com/photos/1334135/20160623/124706.jpg
The leftmost tab is the page this thread is on, the 2nd tab is the Click&Clean homepage that you've been returned to - close that tab with the 'X' to the right, you don't need it any more.
Next - 3 bars > Addons > Extensions - should show you this -
http://media5000.dropshots.com/photos/1334135/20160623/125205.jpg
(that one's also shown smaller than I'd like but it'll have to do, sorry)
You've already restarted F'fox, so the extension is now yours to set as you want via the "Options" box on the right.