Tree Style Tab has noticably increased my efficiency using Firefox. I tried using Vivaldi's with its vertical tabs - that's pretty good, but I find Tree Style Tab to be more polished. I can keep many tabs and thoughts ongoing, yet easily shift topics by hopping from one subtree to another. Recommended.

I get that it's more practical but as long as it is not possible to hide the top bar you end up with double the clutter.

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.

長らく愛用させて頂いてます。
　タブのスクロールバーだけをタブの右側に配置する設定ができれば、大変嬉しいです。
　タブは右側に設置していますが、ツリー構造は左側にあるときと同じ形が好きです。この場合、ページのスクロールバーとタブのスクロールバーが並んでしまい、操作のときに混乱するのです。ご検討いただければ幸いです。

With a tab search it'd be perfect.
And maybe an option to hide the native FF tab bar.

@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.