You could try TabMixPlus: https://github.com/onemen/TabMixPlus
I finally got time to try https://github.com/onemen/TabMixPlus and it worked! Thank you for the suggestion.
It is still developed but you have to enable userchrome.js scripting support to install it.
Hi, commenting here because there's no private messages: I saw your comment about having a browser that still supports TabMixPlus/multirow tabs. If you check the readme on https://github.com/onemen/TabMixPlus, you'll find steps to patch TMP into current Firefox versions. Note that this may impact your security.