I am using Slack from snaps and Firefox from AppCenter (non-curated). I made Firefox my default browser. When I click a link in Slack it opens in Firefox but there are three problems.
- It doesn't open in an active window but creates a new one
- The new window has a strange thick black shadow 
- The new window doesn't share cookies so I am logged out of everything and I have zero extensions so I can't even use my password manager.
 Shadow screenshot https://i.stack.imgur.com/T1kEM.jpg
I actually don't mind the shadow or separate window that much. But I'd love to open the links in an instance of Firefox where I am already logged into various services.
How can I open the links in an instance that has all my cookies and is logged into everything?