If Slack or another website won’t load in Firefox, there is an easy fix. (It sometimes happens with Discourse forums as well.)
If you inspect the failed request in the network console, you might see an error named
To fix it, type this in the address bar to view the registered service workers:
Then search for the name of the site that isn’t loading (probably Slack). Click the “Unregister” button for that site.
That should fix the problem and Slack (or other site) should work again.