gnomey-firefox
A Firefox userChrome.css giving it a header bar and preparing for tree-style tabs.
Firefox 65
Version a05941e finally fully works with it.
The file userChrome.css goes into the chrome folder of your Firefox profile, in my case the full path is /home/paul/.mozilla/firefox/fj57j7ew.default/chrome/userChrome.css (you will likely need to create the chrome folder).
The tab management extension of choice is the great Tree Style Tab by Piro in the Vertigo theme and with a hidden scroll bar.
Since this is hacked-together, if you have a different window button configuration than me you'll have to change the margins a bit.
At the top of the file there are the 2 main things you'll probably want to change - header bar height and margin for the window buttons.
Thanks to the Firefox developers for making this change so easily possible.