I use Firefox Nightly (2017-05-30) and Photon has been teasing us for quite a while.
Turns out that only a few changes are needed to get a pretty close look.
I achieved this enabling the Compact Light theme and adding these rules on my userChrome.css
. I only tested this on Linux.
#TabsToolbar .tab-content {
color: white !important;
}
toolbar:-moz-lwtheme#TabsToolbar, #tabbrowser-tabs,
#TabsToolbar .scrollbutton-up,
#TabsToolbar .scrollbutton-down,
#TabsToolbar {
background: #23327A !important;
-moz-appearance: unset !important;
}