[scns]

In Firefox you can choose for every extension if it is allowed to work in private mode.

[pvorb]

But this ties me to use private mode everytime I visit an important site. This is not what I want.

[ffpip]

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