Get rid of the "Add New Tab" icon on Firefox

What a Hell were they thinking on???

I hate that freaking button (actually, more like a fucking cross really, not even a nice icon, at least on Linux), and it steals screen real state, period.
To get rid of it, open a Terminal, and...

cd ~/.mozilla/firefox/WHATEVER.default/chrome/

And then:

cat > userChrome.css

And then paste this:

.tabs-newtab-button {display: none;}

And hit: 'Ctrl + D', then, re-init Firefox and it is gone for good.