The old built in Firefox tab groups was amazing. I've left a version un-updated on one of my compters for years because of it. Unfortunately nothing has come close to replacing it. Tab groups on Opera is okay but extremely limited. Right now I'm using tree style tabs on another Firefox build and it's not bad. I wish I could name tab groups and that ctrl+tabbing through from the last tab wrapped around to the same group at the end rather than to the next tab group, but other than that it's not bad.

Session Buddy on Chrome is tops.