Oh right, I missed that you were also using fake_statusbar_w_bookmarksbar.css - that's totally not compatible with tabs_below_content_v2.css
You can add this bit to make them behave together:
#PersonalToolbar{
position: static;
grid-row: 11/12;
}
:root:not([inFullscreen]) > body::after{
content: none;
}
Or if you want tabs to be below that fake statusbar then add this as well:
#TabsToolbar{
grid-row: 12/13;
}