source: https://www.pcworld.com/article/823939/vertical-tabs-in-firefox-yes-its-really-possible.html

• open about:config in Firefox tab
• in the search find toolkit.legacyUserProfileCustomizations.stylesheets and toggle value to true
• open about:support
• scroll down to the section that says “Profile Folder,” and click the “Open Folder” button
• Create a new subfolder called chrome within the folder you just opened
• in the new chrome folder, create a new text file userChrome.css
• paste inside css file content from below, save and restart Firefox. Voila! It should work.
  userChrome.css, source: https://raw.githubusercontent.com/OneJaredNewman/firefoxcss/main/userChrome.css

.tabbrowser-tab {
  visibility: hidden;
}
.titlebar-button {
  height: 27px !important;
}
#nav-bar {
  margin-top: -42px;
  margin-right: 140px;
  box-shadow: none !important;
}

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

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

---

Firefox:

119.0.1 (64-bit)

System:

> neofetch
          ▗▄▄▄       ▗▄▄▄▄    ▄▄▄▖            Xtal
          ▜███▙       ▜███▙  ▟███▛            ------------
           ▜███▙       ▜███▙▟███▛             OS: NixOS 23.05.4808.da4024d0ead5 (Stoat) x86_64
            ▜███▙       ▜██████▛              Host: 
     ▟█████████████████▙ ▜████▛     ▟▙        Kernel: 
    ▟███████████████████▙ ▜███▙    ▟██▙       Uptime: 
           ▄▄▄▄▖           ▜███▙  ▟███▛       Packages: 
          ▟███▛             ▜██▛ ▟███▛        Shell: fish 3.6.1
         ▟███▛               ▜▛ ▟███▛         Resolution: 3840x2160
▟███████████▛                  ▟██████████▙   DE: none+i3
▜██████████▛                  ▟███████████▛   WM: i3
      ▟███▛ ▟▙               ▟███▛            Terminal: tmux
     ▟███▛ ▟██▙             ▟███▛             CPU: Intel i5-9400 (6) @ 4.100GHz
    ▟███▛  ▜███▙           ▝▀▀▀▀              GPU: AMD ATI Radeon RX 550
    ▜██▛    ▜███▙ ▜██████████████████▛        Memory: 5795MiB / 48127MiB
     ▜▛     ▟████▙ ▜████████████████▛
           ▟██████▙       ▜███▙
          ▟███▛▜███▙       ▜███▙
         ▟███▛  ▜███▙       ▜███▙
         ▝▀▀▀    ▀▀▀▀▘       ▀▀▀▘