Thanks for the info… hadn’t realised until digging around a few minutes ago that the ‘old tab-centering code’ mentioned by OP YOUBESEENUMBA1 (above) very likely originated from your own reply post here back in May 2023! :)

Try the userstyles suggested in previous r/FirefoxCSS topic: ‘Help needed, how to change border color under the bookmarks toolbar ?’… ignore “under the Bookmarks toolbar” in title… these lines actually appear between main toolbars and the webpage Content window.

BTW - Do give BiggMurr’s reply a try… the whacked-out auto-changing border colours userstyle is a fun option… although you will need black or dark toolbars and content window to really enjoy the show!