The new tab setup has a lack of separation and it doesn't follow the standard Windows assigned colors for title bar and so on. It's called a new Proton interface.

You can go into about:config and "false"-out one or more of the Proton changes. That may help. I "falsed" the single browser.proton.enabled entry and it restored tabs to the way they looked before the latest update.

https://winaero.com/how-to-restore-c...ble-proton-ui/

Going forward, it's getting harder and harder to justify staying with Firefox. Chrome may be the way to go. I just hate to abandon the browser that started it all.