easily the best solution for treed tabs in any browser. it's got some buggy habits but still leaps and bounds better than any solution i've found on firefox or other browsers. i've been using it for several years now and my workflow depends on it.

@Ons-joe, try any of the following CSS codes:

/* Hide Tab Bar contents (Doesn't remove title bar)
#TabsToolbar .tabbrowser-tab,
#TabsToolbar .tabs-newtab-button,
#new-tab-button, #alltabs-button {
display: none !important;
} */

/* Hide Tab Bar contents (Removes title bar) */
#titlebar {
visibility: collapse !important;
}

/* The margin-top so that the buttons and search bars on the nav bar aren't cut-off when Firefox is maximized */
#main-window[sizemode="maximized"] #nav-bar {
margin-top: 8px;
}

/* To hide the sidebar header */
#sidebar-header {
display: none !important;
}

Tree-style tabs is life-changing if you do any kind of work that involves having a lot of tabs open at once. Having them easily grouped by project/subject/whatever you want makes going back to traditional tabs feel like such a mess! I've been a happy user of this extension for years.

Great tool to structure tabs for tab-hoarder like me!
Only one thing could make this better - If the structure would somehow be saved when I use my "OneTab" extension.
If you could work with that developer to get that then this is worth 6 stars :)

Also I like the little pop up message about how how to hide Tree tabs and regular tabs in one step, I wonder if a similar annotation could show new users about the other TST additions/addons?

BTW- Maybe the Close 'Tree Style Tab' button should be a collapse icon (<) instead of (X), because that symbolises a non-reversible closing action to me.
(Version 3.4.8)

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A mandatory Add-on for me. Was better before Quantum update of firefox but with CSS edits you can get some functionality back.