You don't even have to have administrator rights to change Firefox settings, so feel free to change any Firefox settings for your user profile. Change them as you see fit. You currently have Firefox's default userinterface, and it's blue. Change it in Firefox settings as you see fit. There are 3 themes and 3 density available. If that's not enough, upload them. So, click on three dots on right top corner with mouse right key, select Customize and change. In the future, I recommend that you back up your browser settings or sync your browser settings on the web, so there's no problem that you'll need to set all your settings manually every time You reinstall browser... A bit later. PS. it may not have to work for you, namely I checked and I couldn't somehow get such a blue edge. So I even don't imagine, how You have got so blue edge is there. Sorry.