How do I remove this huge and awful bar header?

[SmirnovUA]

How do I remove this huge and awful bar header? I circled it in red just in case ;)

[FurkanKambay]

It's part of Firefox, so it can only be hidden using userChrome.css. Follow the instructions for Option 2 here: https://github.com/mbnuqw/sidebery?tab=readme-ov-file#how-to-hide-native-tabs

The CSS shown there will also hide the native tabs, which you might prefer as well. But if you only want to hide the header, then this is all you need in your userChrome.css:

#sidebar-panel-header {
  display: none;
}

[SmirnovUA]

Extremely grateful, it helped!