A userChrome.css to use with Tree Style Tab (https://github.com/piroor/treestyletab) for Firefox

userChrome.css

/*
Windows
    Location: C:\Users\<YourUsername>\AppData\Roaming\Mozilla\Firefox\Profiles\<YourFirefoxProfile>\chrome
    Notes:
        If minimize, maximize, and close buttons are no longer visible, enable the Title Bar or Menu Bar in Firefox Customize.

macOS
    Location: /Users/<YourUsername>/Library/Application Support/Firefox/Profiles/<YourFirefoxProfile>/chrome
    Notes:
        If minimize, maximize, and close buttons overlap other controls, add the Flexible Space in Firefox Customize.
*/

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);

/* Hide horizontal tab toolbar */
#TabsToolbar {
    visibility:collapse!important;
}

/* Hide "Tree Style Tabs" sidebar header */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
    display:none;
}

Solves issues discussed in piroor/treestyletab#1349 and piroor/treestyletab#1525.

Documentation: https://github.com/piroor/treestyletab/wiki/Code-snippets-for-custom-style-rules

Linux users (usually): /home/<YourUserName>/.mozilla/firefox/<YourFirefoxProfile>/chrome/

about:profiles then show in finder the root folder

Also in Firefox 69+ go to about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true

On new firefox, you need to remove the first @namespace line, otherwise it will not work. See also https://github.com/piroor/treestyletab/wiki/Code-snippets-for-custom-style-rules#for-userchromecss