This gist is to help in setting up the Tree Style Tabs sidebar as a full replacement of the built-in horizontal tabs bar in Firefox. These settings are customized to what I've found to be the most comfortable, but feel free to experiment with them and find the settings that are perfect for you!
This should be added to userChrome.css file under your FireFox profile (on my Macbook, "/Users/<username>/Library/Application Support/Firefox/Profiles/<default profile>/chrome/userChrome.css")
After installing the "Tree Style Tabs" Firefox extension, open the extension settings and paste this into the text box under "Advanced -> Extra style rules for sidebar contents"