Vertical tabs for Firefox, inspired by Edge. Works with Light theme, Dark theme, and System theme (auto dark mode). There are some minor glitches with Alpenglow but they should be easy to fix.

It also plays nice with Container Tabs.

Guarateed compatibility with Firefox 107.

There has recently been in change in Firefox that requires the use of -moz-platform instead of -moz-os-version to use OS-specific CSS. This will work from 99.0b1 upwards but might not on older versions, so be aware of that. If you run an older version of Firefox and have an issue, change this back.

(Also, since the only OS-specific CSS in the file targets Windows, it should not affect you on Mac or Linux either way.)

It should work on newer versions as well unless there’s any breaking changes in how userChrome.css works.

I have only tested this for Linux and macOS. There might be some bugs on Windows because the window decorations look different. I can’t test this because I don’t have a Windows computer, but if you find any bugs and know how to fix it, feel free to submit a PR.

• Go to about:config in your URL bar, search for toolkit.legacyUserProfileCustomizations.stylesheets and set it to true.
• Go to about:profiles in your URL bar, click /”Open Directory”/ next to your Root Directory under your default profile.
• If there is no chrome folder, create it.
• Create a file called userChrome.css inside the chrome folder.
• Copy and paste the contents of userChrome.css into your file (or symlink it).
• OPTIONAL: Adjust --delay setting in the userChrome.css file.
• OPTIONAL: Adjust --fullscreen-sidebar-width setting in the userChrome.css file.
• Install the Tab Center Reborn extension.
• Make sure to enable /”Allow this extension to run in Private windows”/ so you’re not left stranded while browsing.
• Go to about:addons in your URL bar, select Tab Center Reborn, go to Preferences and set:
  • Animations: on.
  • Use current browser theme: on, if you want to use dark mode.
  • Compact Mode: “Enabled” is recommended. It works with “Dynamic” or “Disabled” too but looks nicer with only favicons.
  • Favicon-only pinned tabs: Your choice. If set to “on”, you will only have one icon on the top of the collapsed bar and a row of icons in the expanded state, with “off”, they appear like normal tabs (one per row) with a pin icon on the right. The mod works well with both settings,.
  • Activate Custom Stylesheet and paste the contents of tabCenterReborn.css into the text area below, and click “Save CSS” under the text box.
• Restart Firefox.

• If the sidebar does not show up, you can press Shift-F1 to show the Tab Center Reborn sidebar directly, or Cmd-B / Ctrl-B to toggle the sidebar in general.
• In Fullscreen mode, the sidebar will collapse to a 1px wide border instead to be as unintrusive as possible. You can expand it by pushing your cursor all the way to the edge of the screen. This can be disabled by setting --fullscreen-sidebar-width to 48px.
• If your window controls are not correctly aligned, disable compact mode at Customize toolbar... menu.