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?
That's a very broad question. They have some advantages, like built in ad-blocker (firefox only has tracker blocker) with customizable lists, no analytics except basic user counting.
Firefox in theory has the advantage of being open source though I doubt anyone has independently taken it upon themselves to audit the code base of a whole browser, without payment.