Mozilla Connect

I was a long-time user of Firefox but switched to Edge because it has vertical tabs on one side and a sidebar for links on the other. 

Yes to the expand width and glad to see it is a priority.Being able to have a tree structure (like I have with Tree Style Tab) would be a big plus.

Floorp (based on Firefox) does this.

I would love native vertical tabs. I use Edge more often now because it has vertical tabs natively. The userchrome.css "fix" for removing the horizontal tabs (with Tree Style tabs) is finicky - works on some PC, not others, sometimes needs changing. ...