During triage we discussed this and some points came up: - Currently we are "extending" the titlebar gradient into the tab strip, so that the whole thing looks like one homogeneous titlebar (at least on macOS) - Maybe this will look fine with Photon tab style tabs? (I think it will look bad) - Should we color only the tab strip? - We've seen that sometimes the titlebar is transparent, for example allowing the background to show through for compact themes like Space Fantasy. Can we color the titlebar+tabstrip instead of just the tab strip? So, I started investigating this and ran into an old friend: the bug where setting the drawInTitlebar pref behaves differently when toggled from about:config vs from Customize Mode. Here are some findings on macOS: - The styling of the titlebar text seems to be out of our control as far as I can tell, so we can't use a dark background for it - the text will be invisible. - When the titlebar is toggled from Customize Mode (which is the "common" case) the styling of the background of the titlebar is also out of our control. - When the titlebar is toggled from about:config, the styling of the background of the titlebar IS in our control, and seems to be transparent at the moment, but it can be changed. -moz-window-titlebar works. Considering we seem to be stuck with a dark text color, seems like we can't make the titlebar "vibrant" to go with the tabstrip. So I think the final question to be answered is (in the context of macOS), since the titlebar background has to be light to accommodate the dark text, which is uglier? a) Photon style tabs on the macOS titlebar gradient background (I need to check what it looks like with the titlebar enabled on other platforms) b) A dark tabstrip with the usual macOS gradient titlebar above it.

How about light vibrancy on both the titlebar and the tabs ?

I think we're good with the current look. When I last checked out Photon style tabs on MacOS with titlebar turned on they were broken because of other issues. Now, on Nightly, IMO, they look completely fine. Maybe we should separately track the issue of changing the titlebar text (since it's also needed for dark themes?). Stephen, can you enable the titlebar on your Nightly and give us an opinion on what we should do, given the constraints outlined in comment 1?