I have no idea why that kind of thing would happen, but you might be able to fix it simply be moving the thing downwards a bit:
:root[sizemode="maximized"] #navigator-toolbox{
top: 8px;
}
Although, it also looks like maybe you are using some other styles as well which could be causing the issue, because this image here seems that tabs are kinda abnormal - but it's pretty hard to tell by such small image alone.