Gautham, are you interested to take this?
In order to debug this, besides setting your system to be in High Contrast mode, you need to use our devtools to debug the Firefox UI, which is fun!
On Nightly you can do that by opening devtools (for example by Ctrl+Shift+I/Cmd+Shift+I), pressing the "..." button, Settings, and making sure both "Enable browser chrome and add-on debugging toolboxes" and "Enable remote debugging" checkboxes are checked. Then you can open the Browser Toolbox by pressing Ctrl+Shift+Alt+I. This allows you to remote debug the browser UI using the same tools that you can use to debug web pages. Since this UI is inside a pop-up, from the "..." button in Browser Toolbox select "Disable Pop-up Auto-Hide" which makes sure pop-ups remain open on the window until explicitly closed. Then you can use the Inspector tool to examine what styles are applied to the blue control versus the controls around it and where the style properties are coming from. The most interesting style property you'd want to adjust is the
color property. The fix to this bug is probably either overriding that property somewhere or removing an override that may be being used inappropriately to give us the dark blue text colour.
See bug 1518807 for a similar bug fix (bug in another part of the browser UI) to get some ideas.