Title bar is black and unrendered when turned off

[David Yang]

After creating the value browser.tabs.drawInTitlebar and setting it to true, the tabs are moved up to a bar with the window controls, but the title bar is black and still present. This black bar is still draggable. Also, enabling the flag makes firefox render strangely on startup. These issues only happen in fullscreen mode, windowed mode does not have the black bar, although windowed mode does have a blank spot in the top left corner which I don't know is intentional or not.

Mint 20.2, but I'm using the ubuntu firefox next PPA at https://launchpad.net/~mozillateam/+archive/ubuntu/firefox-next, cinnamon 5.0.5. 750M GPU, Driver Version: 418.113

An imgur link provided for illustration: https://imgur.com/a/Yj9RFIL