This does nothing.

To have multi-row tabs:

1) Download:
https://github.com/Izheil/Firefox-57-full-dark-theme-with-scrollbars

2) Create a directory called "chrome" inside the folder of the profile of Firefox.

3) Copy the following files into that folder:
Multirow and other functions/Multirow tabs/MultiRowTabLiteforFx.uc.js
Multirow and other functions/Multirow tabs/userChrome.css
Multirow and other functions/Multirow tabs/userChrome.xml

Sometimes, after updating Firefox, the multi-row does not work anymore.
I just re-download the current version of the above great piece of work.