I noticed this is only happening when the window is maximized AND the "Tabs on Top" setting is on. Hope this helps and can be resolved in future releases.

Does it help if you set the pref browser.tabs.drawInTitlebar to false on the about:config page.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

Yes, that helps by dropping the tabs below the title bar, like they are when the window is not maximized. It's a fine temporary solution, but would still be interested in seeing this issue resolved permanently later on down the road, as it's nice to have as minimal a header area as possible. Thanks!