Version 0.8.3 is available at the moment.
You can either:
If you want to install it right away, Firefox might give you the following message:
If that happens, it means you need to tell Firefox to allow installation from this website. When you click on the "Edit Options..." button, Firefox will present you with a dialog displaying a list of all the websites from which it is allowed to install extensions.
You need to add the website www.mercille.org to the list of allowed sites in order to install the extension (and any other extensions from this website) without having to save the file to your local drive first.