Tab Mix Plus is a very popular legacy extension for the Mozilla Firefox browser that enhances Firefox's tab browsing abilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more.
Session Manager feature is not included at the moment since it requires complete rewrite.

If you are interested in keeping Tab Mix Plus working, please donate.

• Installation
• Firefox 101 update
• Browser Compatibility
• Configuration
• Docs

1. Legacy extensions loader
   Since Firefox removed the internal component that loads legacy extension, in order to install Tab Mix Plus, or any other legacy extension, you have to install 3rd party helper script/extension.
   The developer of these script/extension recommend using the first option:
   • If you're using normal Firefox (aka "stable" or "release"), Beta or ESR, you need to install this script userChromeJS.
   • bootstrapLoader.xpi extension is an option for those who use Firefox Developer Edition or Firefox Nightly.

2. Download XPI
   Download xpi file from our releases page to your local computer.

   If you are using Firefox Beta, Developer Edition or Nightly we recommend using the latest Tab Mix Plus development build (tags with pre in the title)

3. Install XPI
   To install the file you have just downloaded:
   • Open Add-ons Manager tab (about:addons) and select Extensions.
     or
     Click the menu button ☰, click Add-ons and Themes and select Extensions.
   • To add the downloaded add-on to the list of available add-ons, drag and drop the file into the Add-ons window. The add-on is added to the list.
   • The installation process should begin.

After updating to Firefox version 101 or newer, you have to reinstall userChromeJS and Tab Mix.

• Reinstall latest version of userChromeJS.
• Open about:support and click "Clear startup cache…" to force Firefox to load userChromeJS on the next startup.
• Reinstall latest Tab Mix Plus again.

• Firefox 78 - Firefox Nightly
• Waterfox G3

For Pale Moon download Tab Mix Plus from here

Tab Mix Plus comes with Options window that includes all of its preferences as well as adding user interface to important Firefox hidden preferences. It is recommended that you make all changes to the preference in the options window. The options window is available from the Add-ons Manager or from the Tools menu (unless you turn this option off).

Read The Docs