Here's the one I used:

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/autohide_sidebar.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* Show sidebar only when the cursor is over it  */
/* The border controlling sidebar width will be removed so you'll need to modify these values to change width */

#sidebar-box {
 --uc-sidebar-width: 60px;
 --uc-sidebar-hover-width: 210px;
 --uc-autohide-sidebar-delay: 100ms; /* Wait 0.6s before hiding sidebar */
 position: relative;
 min-width: var(--uc-sidebar-width) !important;
 width: var(--uc-sidebar-width) !important;
 max-width: var(--uc-sidebar-width) !important;
 z-index: 1;
 background-color: #110a0d !important;
}

#sidebar-box[positionend] {
 direction: rtl;
}
#sidebar-box[positionend] > * {
 direction: ltr;
}

#sidebar-box[positionend]:-moz-locale-dir(rtl) {
 direction: ltr;
}
#sidebar-box[positionend]:-moz-locale-dir(rtl) > * {
 direction: rtl;
}

#main-window[sizemode="fullscreen"] #sidebar-box {
 --uc-sidebar-width: 1px;
}

#sidebar-splitter {
 display: none;
}

#sidebar-header {
 overflow: hidden;
 /* color: var(--chrome-color, inherit) !important; */
 padding-inline: 0 !important;
}

#sidebar-header::before,
#sidebar-header::after {
 content: "";
 display: flex;
 padding-left: 8px;
}

#sidebar-header,
#sidebar {
 transition: min-width 115ms linear var(--uc-autohide-sidebar-delay) !important;
 min-width: var(--uc-sidebar-width) !important;
 will-change: min-width;
}
#sidebar-box:hover > #sidebar-header,
#sidebar-box:hover > #sidebar {
 min-width: var(--uc-sidebar-hover-width) !important;
 transition-delay: 0ms !important;
}

.sidebar-panel {
 /* background-color: transparent !important; */
 /* color: var(--newtab-text-primary-color) !important; */
}

.sidebar-panel #search-box {
 -moz-appearance: none !important;
 /* background-color: rgba(249, 249, 250, 0.1) !important; */
 color: inherit !important;
}

/* Add sidebar divider and give it background */

#sidebar,
#sidebar-header {
 background-color: inherit !important;
 border-inline: 1px solid rgb(80, 80, 80);
 border-inline-width: 0px 1px;
}

#sidebar-box:not([positionend]) > :-moz-locale-dir(rtl),
#sidebar-box[positionend] > * {
 border-inline-width: 1px 0px;
}

/* Move statuspanel to the other side when sidebar is hovered so it doesn't get covered by sidebar */

#sidebar-box:not([positionend]):hover ~ #appcontent #statuspanel {
 inset-inline: auto 0px !important;
}
#sidebar-box:not([positionend]):hover ~ #appcontent #statuspanel-label {
 margin-inline: 0px !important;
 border-left-style: solid !important;
}`___`