I use an addon to display my tabs in a tree on the side of the screen. As such, the horizontal tabs at the top of the screen are unnecessary, and I would like to remove them. I've tried to follow a few other guides on how to do this, without success. I've created a userChrome.css file in the "chrome" folder of my active Firefox profile. It currently looks like this: #tabbrowser-tabs { visibility: collapse !important; min-height: 0 !important; } tab{ display:none!important; } However, the tabs remain. I've tried a few other permutations of these rules. I've made sure to save the text file and restart firefox multiple times. It also persisted across a PC restart. What am I doing wrong?