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.
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)