Ok, note to self; after a while, having slack to open all links in Firefox, when all your authentication is set up for Chrome, well, it wears a bit thin. Documentation on this front varies depending on your desktop flavour, but if you're running the native Slack application on KDE, on Linux, then the command you are looking for is;

xdg-settings set default-url-scheme-handler https google-chrome.desktop

Obvious really, but for the non physic, this will sort the problem, and you won't even have to restart anything :)