Firefox Config

tab-center-redux.css

/* Hide the New Tab button */
#newtab {
    display: none;
}

/* Remove extra padding and margins from Search tabs field */
#searchbox {
    padding: 0px;
    margin: -2px;
}

/* Shrink tabs and grow active tabs in compact mode */
#tablist-wrapper.shrinked .tab.pinned {
    height: 20px !important;
}

#tablist-wrapper.shrinked .tab:not(.pinned) {
    height: 24px !important;
}

#tablist-wrapper.shrinked .tab.tab.active {
    height: 32px !important;
}

/* Auto-hide scroll-bar with transition */
.tab {
    max-width: 100vw;
}

#tablist {
    margin-right: -15px;  transition: margin-right 0.2s;
}

#tablist:hover {
    margin-right: 0px;
    transition: margin-right 0.2s;
}

#tablist-wrapper {
    overflow-x: hidden;
}

userChrome.css

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

/* Hide horizontal tab bar
#main-window:not([customizing]):not([tabsintitlebar="true"]) #TabsToolbar {
    visibility: collapse;
}

#navigator-toolbox {
    margin-top: 1px;
} */

/* Show horizontal tab bar when menu bar is visible
#toolbar-menubar[inactive="true"] + #TabsToolbar {
    visibility: collapse !important;
} */

/* Sidebar min and max width removal */
#sidebar {
    max-width: none !important;
    min-width: 0 !important;
}

/* Reduce sidebar header height */
#sidebar-box #sidebar-header * {
    margin-top: -3px !important;
    margin-bottom: -3px !important;
}

/* Hide splitter, when using Tree Style Tab or Tab Center Redux. */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] + #sidebar-splitter,
#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"] + #sidebar-splitter {
    display: none !important;
}

/* Hide sidebar header, when using Tree Style Tab or Tab Center Redux */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header,
#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"] #sidebar-header {
    visibility: collapse;
}

/* Shrink sidebar until hovered, when using Tree Style Tab or Tab Center Redux. */
:root {
    --thin-tab-width: 30px;
    --wide-tab-width: 300px;
}

#sidebar-box:not([sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"]),
#sidebar-box:not([sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"]) {
    min-width: var(--wide-tab-width) !important;
    max-width: none !important;
}

#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"],
#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"] {
    position: relative !important;
    transition: all 100ms !important;
    min-width: var(--thin-tab-width) !important;
    max-width: var(--thin-tab-width) !important;
}

#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"]:hover,
#sidebar-box[sidebarcommand="_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action"]:hover {
    transition: all 200ms !important;
    min-width: var(--wide-tab-width) !important;
    max-width: var(--wide-tab-width) !important;
    margin-right: calc((var(--wide-tab-width) - var(--thin-tab-width)) * -1) !important;
}

/* Sidebar header dark theme fixes */
#sidebar-header {
    background: #0C0C0D;
    border-bottom: none !important;
}

#sidebar-splitter {
    border-right-color: #0C0C0D !important;
    border-left-color: #0C0C0D !important;
}

#sidebar-switcher-target,
#sidebar-close {
    filter: invert(100%);
}

/* Move sidebar header to the bottom
#sidebar-box {
  -moz-box-direction: reverse;
}

#sidebar-switcher-arrow {
  transform: rotate(180deg);
} */

Lines 17 to 21 (userchrome.css) do not seem to work anymore with FF 107.

I can not reduce the size of the sidebar. I can still expand but not reduce.

Could you help please ?

Edit : Solution : Use #sidebar-box instead of #sidebar since that is the element with max and min width set.

Thanks to It_Was_The_Other_Guy on Reddit