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?
Not exactly what you are looking for but Sidebery has a feature called Snapshots that allow you to save a group of tabs in their current state to open them later.
That's more of an alternative to vivaldi sessions, not workspaces
Interesting. I'll need to test it to see how it exactly behaves.