Holy shnikies, a Firefox Echelon user, who's using it on Windows 11? I'm using the same browser right now on Window 7.
Yeah, that little blue line on the left/bottom edge originated with Windows Vista, and is being done by your custom theme. Maybe you tried to get a Windows 7 theme, but they lied to you, it's clearly a Vista theme. I'm actually looking into editing my Windows 7 system so that it includes this accent, but that's besides the point - there's nothing inside of Echelon, or Windhawk, that's gonna help you. The thing that's annoying you is a 20+ year old tiny bit of hard-coded color code, it isn't dynamic or changeable by anything else.
You'd need to inspect your custom theme, and maybe compare it against the default windows 7 aero theme, there's gonna be a line of code somewhere that controls this. Resource hacker and research are your friends here, but finding another theme might be easier.
Good luck dude, what a beautiful thing to see running on 23H2, respect!