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

/* For usage with treestyle tabs */
sidebar-main {
  width: 0px;
}

/* 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: 10rem;
  --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: relative !important;
  z-index: 10 !important;
  overflow: hidden;
  min-width: var(--thin-tab-width) !important;
  max-width: var(--thin-tab-width) !important;
  height: 100%;
  transition: all 500ms ease !important;
}
#sidebar-box[sidebarcommand='treestyletab_piro_sakura_ne_jp-sidebar-action']:hover {
  transition: all 500ms 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: 99;
}

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

/* Ensure TST's content width resizes with the sidebar */
#sidebar-box[sidebarcommand='treestyletab_piro_sakura_ne_jp-sidebar-action']
  .treestyletab-contents {
  width: 100% !important;
  transition: width 200ms ease !important;
}

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

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

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

Now restart firefox and enjoy your vertical tabs!