Your UI config tip for FF doesn’t work

Your tip in this article about changing the width of the tab, doesn’t work for me.
What am I doing wrong?

***browser.tabs.tabMinWidth, double-click the Value and change it to something you like. In my case, 110.***
I’ve changed this to 50 but the default stays.

http://www.zdnet.com/article/how-to-tweak-the-new-firefox-57-quantum-browser-to-suit-your-preferences/