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?
By tab stacking you mean that feature where you can create those collapsible tab groups, right?
It's different. Instead of grouped tabs that expand within the same row of tabs, tab stacks show the group in a second row of tabs. Works great if you have a lot of tabs that tend to shrink in width to much to be usable.