Is Firefox Relay available for Macbook?

Yes, it should be available and you don't need an extension for it. This extension is now in maintenance mode and its features have been transferred to core Firefox. Just go to Settings -> Privacy and Security -> Additional protection and select Suggest Firefox Relay email masks.

You can manage your masks in https://relay.firefox.com.

I get a blank screen when I go to https://relay.firefox.com. What's the problem? Thank you!

I don't know, it should work. Try today.