Firefox opening applications and links problem

Firefox is set to be my default browser - I made the setting in Windows default apps.
Firefox appears in the Comodo Widget along with Chrome. I used CruelSister’s settings to make Chrome run in a sandbox.

One problem that I face is that when I click on links in Thunderbird messages, the related pages open in a non-contained Firefox. Also if I right click on the Plex server icon on the taskbar then Plex too opens in a non-contained Firefox. I say non-contained because I do not see the green border and I hope that I’m right about this. Each non-contained Firefox instance is different.

How can I set Firefox to always open as a contained app and also for links from other sources or apps (like Plex server) to also open in a contained Firefox? Preferably I would want only one instance of Firefox to be running.