The Tab Center makes the browsing experience a little bit strange. As the left panel is part of the Firefox window and people is usually reading in a Z shape, the white background is somehow “out of context”.
I am siggesting here something like what Vivaldi is making: choose a color from the page being shown and use it to paint the background of the tabs panel. For example, in this page, making the Tab Center background color red would make Tab Center not so part of the chrome.
Yeah, but I think that (like the regular tabs), Tab Center is part of the chrome, and that should possibly be reflected in the UI. I’ll think about it a little more, though. Thanks for the suggestion!
Ok. You could also take it like a suggestion for regular tabs, then.
Yeah, it is a neat idea… I might look into it in my spare time. 
Looking for any add-on which could add any feature from Vivaldi into Firefox, I have just found VivaldiFox. It colours most of the interface like does Vivaldi.
https://addons.mozilla.org/en-US/firefox/addon/vivaldifox/?src=search
It is released as MIT/X11 by Tim Nguyen. If anyone would like to try to add this feature into Tab Center, maybe it could worth take a look into it.
I stoppped using Tab Center, althought I am still -and will keep- using Activity Stream Test Pilot as long as I can, but reading some comments from the add-on page, it seems that doesn’t work for Tab Center.
Maybe what is needed is that Tim Nguyen “fixes” it.
(EDIT: I forgot to add the add-on link)
I agree. Adding a color would clearly show that the tab panel is separate from the main page. I find myself always trying to scroll the main page when the cursor is in the tab panel. A separate control bar would be good too. But some selectable color or color intensity or even “theme” would still help. And look nice too. I would suggest the same use of color for the background of the selected tab and the tab bar too… How the tab bar looks does affect how well it works.