add MacOS Firefox Theme

[ryanfortner]

I found this project, which is a theme for Firefox that looks like Safari, I think it would be a good addition to the theme converter?

(To install the latest firefox you can use the instructions here)

[Itai-Nelken]

I used https://github.com/vinceliuice/WhiteSur-gtk-theme/tree/master/src%2Fother%2Ffirefox on the firefox in the repos and it worked fine.

[techcoder20]

Good Idea This will be a nice addition to the theme. I will try to add it :)