Introduction

Microsoft Edges vertical tab feature is frankly speaking amazing and it is a shame we currently don’t have an equivalent implementation in firefox. The follwing guide shows you how to achieve it anyways.

Spoiler alert: It looks complicated, but is actually pretty easy.

Install a Sidebar Tabs Extension

First search for a firefox extension that displays tabs in the sidebar at https://addons.mozilla.org

The one I am using is called Tree Style Tab

Disable Firefox Menu Bar

1. Right click on the menu bar
2. Uncheck menu bar

Enable Stylesheet Customization

1. Go to about:config
2. Set the flag toolkit.legacyUserProfileCustomizations.stylesheets to true

Locate Profile Directory

1. In firefox type about:support in the address line
2. Search for the field Profile Directory
3. Open this directory in your file explorer and create a folder named chrome
4. Create a file named userChrome.css inside the chrome folder with the following content

your-profile-directory/chrome/userChrome.css

/***** Hide tab top bar *****/

.tabbrowser-tab {
  visibility: collapse;
}
.titlebar-button {
  height: 30px !important;
  margin-top: 5px !important;
  margin-bottom: 5px !important;
}
#nav-bar {
  margin-top: -40px;
  margin-right: 175px;
  box-shadow: none !important;
}

[uidensity="compact"]:root .titlebar-button {
  height: 32px !important;
}
[uidensity="compact"]:root #nav-bar {
  margin-top: -40px;
}

#titlebar-spacer {
  background-color: var(--chrome-secondary-background-color);
}
#titlebar-buttonbox-container {
  background-color: var(--chrome-secondary-background-color);
}
.titlebar-color {
  background-color: var(--toolbar-bgcolor);
}

#main-window[inFullscreen="true"] #sidebar-box,
#main-window[inFullscreen="true"] #sidebar-box + splitter {
    visibility: collapse;
}

#sidebar-header {
  display: none !important;
}


/***** Auto hide vertical tabs *****/

/* Hide main tabs toolbar */
#main-window[tabsintitlebar="true"]:not([extradragspace="true"]) #TabsToolbar > .toolbar-items {
    opacity: 0;
    pointer-events: none;
}
#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
    visibility: collapse !important;
}

/* Sidebar min and max width removal */
#sidebar-box {
    max-width: none !important;
    min-width: 0px !important;
}
/* Hide splitter, when using Tree Style Tab. */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] + #sidebar-splitter {
    display: none !important;
}
/* Hide sidebar header, when using Tree Style Tab. */
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar-header {
    visibility: collapse;
}

/* Shrink sidebar until hovered, when using Tree Style Tab. */
:root {
    --thin-tab-width: 12rem;
    --wide-tab-width: 24rem;
}
#sidebar-box:not([sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"]) {
    min-width: var(--wide-tab-width) !important;
    max-width: none !important;
}
 
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] {
    position: absolute;
    z-index: 1;
    overflow: hidden;
    min-width: var(--thin-tab-width) !important;
    max-width: var(--thin-tab-width) !important;
    height: 100%;
    transition: all 200ms ease !important;
    
}
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"]:hover {
    transition: all 200ms ease !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;
    z-index: 1;
}

#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] ~ #appcontent {
    margin-right: 0;
    margin-left: var(--thin-tab-width);
  }

/* Full screen */
#main-window[inFullscreen] #appcontent {
  margin-left: 0;
  margin-right: 0;
}

NOTE:
In the above code, you can adjust the values of the follwing three variables to whatever suits you best.

:root {
    --thin-tab-width: 12rem;
    --wide-tab-width: 24rem;
}

Now restart firefox and enjoy your vertical tabs!