Vivaldi (and Edge) have this absolutely wonderful capability that allows me to split one tab into two or four at the same time.
At least in my workflow it's quite useful because I usually work with several tabs open and sometimes two related tabs (say, a document I'm reading and a document I'm replying to according to that one), I know that I can perfectly have another Firefox window open next to it and fulfill that function, but I wish I could do it directly from Firefox.
Does anyone know of an add-on that fulfills this purpose? Or maybe a dev who is developing it?
If you need 2+ tab stacks, I think you would rather need a tab group manager, like Simple Tab Groups. It does not support nested groups, but you may be able to imitate it with "folder names" in the group name, or there might be better such addons too.
Honestly, I don't see how 2+ rows of tabs (+stacks) would fit on screen