On Firefox

How to install CSS Pro on Firefox:

Step 1. Download and unzip the "CSS Pro.zip" file from your Gumroad's receipt or library.
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).‌
To manage your license activations in the future, use the My License page.
Here's a video version of this tutorial: