New versions of Firefox removed some support for extensions that allowed multiple tab rows.

Here is how you can enable it:

  1. Type about:support in url field of firefox, hit enter.
  2. On the ‘Profile Folder’ setting (about 8th from the top) click the ‘Open Folder’ button.
  3. Create a ‘chrome’ subfolder, if you don’t already have one.
  4. Inside that folder create a file called ‘userChrome.css’ (right click > new > text document – make sure to name the file userChrome.css and not userChrome.css.txt or something)
  5. Firefox 65 and newer:
    Open the file for editing and paste the contents of this file: https://github.com/MrOtherGuy/firefox-csshacks/blob/master/chrome/multi-row_tabs.css 
    Versions before Firefox 65:
    Open the file for editing and paste the contents of this file:
    https://gist.githubusercontent.com/forexhill/b0446cc31e5001c9e87754df83f0f1ca/raw/f4f1a807d43434a1f10908364dec56ecdf08422c/gistfile1.txt
  6. Restart FF and you should have multi row tabs

After Firefox 65 method comes from here

Before Firefox 65 method comes from user fh1 at this link

Firefox 58 Quantum resulted in our old extensions not working anymore. But if you want to enable extensions and keep using them, here’s what you need to do:

  • Download and install Firefox Nightly from here.

  • Start Firefox
  • Type about:config in address bar
  • and set the preference ‘extensions.legacy.enabled‘ to true
  • Restart Firefox.

You should now be able to enable multi-process capable extensions: