Install the theme directly from Firefox Addons.
If you are a git user, you can install the theme and keep up to date by cloning the repo:
$ git clone https://github.com/dracula/firefox.git
Download using the GitHub .zip download option and unzip them.
Get updates about this theme and more. 5,898 people enjoy it.
This theme wouldn't exist without these people. Wanna help too? Check the repository on GitHub.