For me it's the Tab Groups add-on which is just irreplaceable! Firefox's "Containers" helps a bit but not that much.
Between Tab Groups and Tab Mix Plus for a multi-line tab bar I have to seriously redesign my workflow.
I could live with Tree Style Tab if it could nest tabs on the horizontal tab bar. I run FF on a portrait 1200x1920 screen and the vertical sidebar takes up too much space.
I cannot stress enough how it helps me to switch between contexts. It's one of those things that you just did not know that you needed so much.
What options do we have?