Practically essential if you open many tabs. Hierarchical indentation is what makes this superior to all other solutions.

The transition to Web Extensions is excellent considering the situation. There is a current bug on Firefox's end that keeps the horizontal tabs around. You can address this by creating a file in your Firefox profile, \chrome\userChrome.css and add the following rules:

/* Remove horizontal tabs */
#tabbrowser-tabs {
visibility: collapse;
}

If you want to get rid of the sidebar header, add this to the file:

/* Remove sidebar header */
#sidebar-header {
visibility: collapse;
}

You will have to restart Firefox to enact the changes.