I only discovered it because I was going to add a similar feature to my Chrome extension, and I was researching to see how others tend to implement it. I was glad to see that Chrome offers the feature natively, and surprised to see that Firefox didn’t.

You can add domains to protected sites. No addons will work there.

So if you want no extension to be able to read gmail,

Add mail.google.com or google.com to

extensions.webextensions.RestrictedDomains in about:config

It's fairly new (maybe 1-2 years old, I forget), that's probably why you didn't notice it.