What it basically does is removes all the close buttons. So you will have to use CTRL+W to close tabs now if that's what you want.
What I was looking for was a way to get one close button at the right hand side of the browser window for all tabs (Opera style), because I find I click the close buttons by mistake all the time and have to strain my eyes to prevent it.
I found this by subscribing to Mozilla's planet blog feed, and the author's April 2014 blog about how he put this extension together at dirkjan.ochtman.nl is really informative and gave me great confidence in the quality.
Well, so I fixed this unjustified design decision with a single click (no restart nor enabling required; just click "install addon" and the buttons were gone). I would have preferred that the option never went away though.
I've been raging over the removal of the option to hide all the close buttons for days. Never actually stopped to think that an add-on would restore it, but here we are. This is the first I noticed, and it does indeed work. Thank you.