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.
Receive updates about this theme and more about Dracula.
This theme wouldn't exist without these people. Wanna help too? Check the repository on GitHub.