Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
In Windows 10 (and perhaps lower), there's a user setting for putting an accent colour in the title bar of windowed applications.
For proper integration, we should detect that setting and colour our title bar appropriately.