Thank you! It works exactly as intended!

You are right, it was the height adjustment I had to make for removing the line beneath toolbar, it was causing a blank space (not a border). Adjusting the height solved it. Thank you.

Thank you again.

Thank you. It works! As for this, is reducing the --uc-bm-height the only way?

One more thing is I have removed the line underneath tool bar with

#navigator-toolbox {
    border-bottom: none !important;
}

This is causing 1 pixel gap between the bookmark bar and toolbar. How can I fix it?

Thank you, the workaround with widget.gtk.ignore-bogus-leave-notify works. And yes the temporary fix was kind of weird, as it used to work if I launched firefox directly, but not when it was started by clicking some link in other applications.

I see. Thank you for the explanation as well!

This worked, thank you. Would you mind telling what difference between using urlbar-background over the other one?

Thank you, it worked.