New versions of Firefox removed some support for extensions that allowed multiple tab rows. Here is how you can enable it:
- Type about:support in url field of firefox, hit enter.
- On the ‘Profile Folder’ setting (about 8th from the top) click the ‘Open Folder’ button.
- Create a ‘chrome’ subfolder, if you don’t already have one.
- 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)
- Open the file for editing and paste the contents of this file;
- Restart FF and you should have multi row tabs
This comes from user fh1 at this link