I have started using Ubuntu 18
When I click on a link in Slack, new Firefox opens. And in dock/panel, it shows as child application of Slack. This causes them to be grouped together. When ungrouped in panel, the Firefox still shows under Slack and has Slack icon. I've noticed similar behavior when opening an application from different application. So it is not a problem of Slack or Firefox, but general Ubuntu UI problem.
Is there any way to change this behavior so Firefox opened from Slack shows up as Firefox application itself?
Did you install Slack using snap?
I had this problem and removed the snap install
sudo snap remove slack
then reinstalled from the .deb download from here: https://slack.com/intl/en-nz/downloads/instructions/ubuntu
sudo apt install ./slack-desktop-4.7.0-amd64.deb
and the problem went away.
I can't pretend to understand why(!) but it fixed the problem for me.