How to open links from Slack in existing Firefox window

Question

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.

1. It doesn't open in an active window but creates a new one
2. The new window has a strange thick black shadow [1]
3. 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.

[1] Shadow screenshot https://imgur.com/a/STApbkU

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?

Answer 1

I've found that the problem was Slack from Snap. When I installed both Slack and Firefox via AppCenter as non curated apps, it works normally. Something is not working between different stores.