Running 18.04 LTS I discovered an odd behavior from Slack. Instead of opening links in my regular FIrefox installation, it opened its own Firefox-instance. That instance had no plugins and was grouped as a slack process in the menu bar. Weird.
Reinstalling Slack from Ubuntu Software didn't help and I couldn't find a setting within Slack that controlled this.
The solution to my issue was to install the .deb file from Slack's homepage, instead of using Ubuntu Software. After doing so, links opened through my regular Firefox installation, instead of starting new processes belonging to Slack.
Source: askubuntu.com
