I, like a fair few other Firefox users (at least, I think) use Firefox maximized on it’s own screen. When it’s maximized like this, the tab bar is automatically set to be the title bar, saving on screen space. While I’m interested in Tab Center, currently enabling it when maximized moves the tabs over to the left, as expected, but leaves the window titlebar empty, rather than moving the main toolbar with navigation and the like up, meaning there’s dead space on the screen.
If possible, I think the main toolbar should move into the space the (now un-needed) tab bar vacated, to make the browsing experience more streamlined and to make better use of the available screen real-estate. As I already have tab groups functionality (with it’s requisite searching capability), this is the main sticking point for me as a user. Solve this issue, and I think you’ll have a convert on your hands.