One still has to enter a mobile number to sign up for that Google account. But the larger difference is, that account would still be a standalone email address,

One still has to enter a mobile number to sign up for that Google account. But the larger difference is, that account would still be a standalone email address, which just happens to forward to your main Gmail address. But Firefox Relay (and similar products, like AnonAddy and SimpleLogin) are alias services. The idea with these services is to create addresses that can be immediately blocked, if they get into the hands of spammers. I am a happy SimpleLogin customer, and have made as many as 200 addresses. AnonAddy is a great start too, for those that need unlimited addresses. Both allow responding from those addresses, while AnonAddy's count is less.