This is only icon theme and not contained GTK UI style. If you wanna mess around the styles and change something, you might find these things useful. To use the Inspector to debug the UI, open the ...

Firefox Nightly now lets you create, Switch & Manage profiles like Chrome with themes, avatars to seperate work and personal ...

Navigate to Preferences by entering about:preferences in the browser’s address bar or click the gear icon and got to ... Meanwhile, Firefox 77.0.1 is Mozilla’s latest stable release.