Can I trouble someone for help please? :)

Do you have the DOM Inspector extension? You can use that to examine "Chrome" documents, which allow you to poke around on the toolbar and see, for example, which element has a border, or maybe it's not a border but part of the background.

https://addons.mozilla.org/firefox/addon/dom-inspector-6622/

You can access DOM Inspector from the classic menu bar under Tools > Web Developer. Its legacy shortcut of Ctrl+Shift+i has been superseded by the built-in Inspector.

I probably won't be on a Windows 10 system for several days, so hopefully you'll figure it out before then.

Thanks. Can't figure out how to use DOM inspector. I have the window open, but it's different from the regular built in inspector which I use daily.

On the DOM Inspector's File menu, use Inspect Chrome Document to select the current browser window. (It's easier when you don't have a lot of windows open.) Then you can use click-to-select to inspect the toolbar.

Thanks. I got that far, but still can't tackle that line on the bottom.

I've tried:

#navigator-toolbox {
    background: rgba(23, 23, 23, 100) !important;   
    border-bottom: 0px !important;
}

#nav-bar {
    background: rgba(23, 23, 23, 100) !important;   
    border-bottom: 0px !important;
}

None seem to fix that line

Ok, found a solution on userstyles's site

1. navigator-toolbox::after{

background-color: #000 !important; }

Thanks!