Inside it, you'll find the files README, Chrome.url, Firefox.xpi, and a .dmg file:
The "Firefox.xpi" is the one you'll need
Step 2. Drag and drop the "Firefox.xpi" file into your Firefox browser window, and click on the button "Add" to install the add-on.
Now that you have the extension installed, it's time to activate it with a license key, which you've got in your receipt, but can also be retrieved here if you forgot/lost it.
Step 3. Open any website on a new tab, for example, Google, or your own website.
Step 4. Click on CSS Pro's icon on your toolbar to open the extension:
A modal (popup) will appear asking for the license.
Step 5. Input your license key and validate it by clicking on "Check license key":
🎉 Done! Your license is now activated, and you can use CSS Pro anywhere else.
The extension will not ask for your license again (unless deactivated or disabled).
Here's a video version of this tutorial: