I work in the Software Security field. One of my biggest paranoias is that a rogue site will be able to access one of my protected sites, such as banks or email. In the past, I was able to mitigate this by using different profiles in Chrome, but that required separate browsers to be running. Firefox had profiles, but I couldn't open them simultaneously, which made them useless. Now with Firefox containers, I am opening my email in separate contexts from my shopping, and other contexts. I have separate containers for different roles in trusted platforms. The big win is that I can set defaults for my sites. For instance, Amazon always opens in my shopping container. Sweet.

The only reason I cannot give this five stars is because of this one criticism. Links from my email open by default in the same container as my email. This is a security shortcoming because it opens me up to phishing attacks. You can work around using the context menu and opening in a specific container, but I would rather it be a default.