This bug is not currently tracked.
User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:67.0) Gecko/20100101 Firefox/67.0
Steps to reproduce:
"Security Error: Content at resource://activity-stream/css/activity-stream.css may not load data from blob:null/e6828ba1-a339-4da9-8d39-[redacted]." is spamming the browser console.
The error shows up in bug 1490213 comment 3, etc. Over there, it was triggered by clearing history