[Mozilla Enterprise] Opening link from other program opens new instance of firefox

[Timo Pietilä]

This is behavior of 68 that is not specifically esr-related but I would 
like to know how to prevent it:

If you click a link from for example thunderbird it doesn't open link in 
already open FF but to new instance of FF. How do I prevent this? Is 
there a property or policy that forces links to open in new tab in 
already open FF?

Timo Pietilä