Many users don't like it adwaita, the default theme of Gnome 3However, as is logical, others if they like it and the good news is that we can use the appearance of this theme in Firefoxapart from gtk theme that we have activated.
You just have to download the extension from this link:
https://addons.mozilla.org/es/firefox/addon/adwaita/
I generally like the way it looks a lot, especially the eyelashes. I just have to see how I modify the theme to narrow the scroll bars a bit.