@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.