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.

Kiepsko ! ! ! - Ponieważ nie zawiera wielu - wielu WSPANIAŁYCH funkcji, do których przez wiele lat się przyzwyczailiśmy które były dostępne w "TAB MIX PLUS" za którym bardzo tęsknimy. :(

Es gibt Menschen wie mich, die aus bestimmten Gründen FF56.0.2 oder älter benutzen. Diese werden hier vor den Kopf gestoßen. "Tab Mix Plus" wurde gekillt und "Tab Mix Link" so beschnitten, dass es erst ab FF57.0 funktioniert. Äußerst unfreundlich ist das.

Finally found an add-on that can easily set how my tabs open instead of trying to search through about:config
Best suited add-on for my needs!

It used to be very good a few years ago. Many options. Today, I only use it to open All Links in a New Tab; however, it does not work on toolbar bookmarks on Firefox opening in a new tab; I had to edit about:config. For all Youtube links I have to use Tampermonkey. And there are already other options to open all links in new tab.

I loved TMP since years and I am really sad that it isn't working anymore like lots of other useful addons

So sad to down-rate this app!
I have loved this extension, and it's previous incarnations, for YEARS!
Firefox pays NO ATTENTION to the settings... Waterfox does MUCH better.
Keep up the good works, dev team! Hopefully, Firefox will get their sh-- together... someday...

UPDATE! 8-11-20
This doesn't seem to be working well in either firefox or waterfox. I'm so sad. There used to be a popup window with all these choices, but now it's a short page that doesn't seem to make a difference in either of my browsers.
I've loved this extension for years and every once in a while I'll get my old option window back, but I've recently switched laptops (hey, it could be the laptop - what do I know!) and it really isn't working for my browsers, right now. :(

this add-on is truly one that really works for web devs like me. my only wish is that the day will come soon that the full functionality of the legacy extension will be brought back to this new version.

5 stars for the old Tab Mix Plus extension, which was my must have. Currently not much functionality, but I hope... that day will come.