Google Chrome recently launched a new Material Design theme. If you like it, you can also get it for Firefox with MaterialFox.
MaterialFox works with all three color schemes in Firefox; default, light and dark.
MaterialFox’s theme is so good, you can hardly tell the difference. Better yet, you still have Firefox under the hood.
The install process is pretty easy, but it’s not as easy as just installing an addon. The instructions are pretty straight forward though and you’ll have MaterialFox up and running in no time.
If you do install it, check GitHub often as it’s under active development and keeps getting better all the time.