Some time ago, I discovered a bunch of background update files in my temp directory:
In each directory there was 2 empty database files:
229,376 cert9.db
294,912 key4.db
I found this very strange, as Firefox was set to not check for updates. Searching for answers did not produce any results. Then I noticed that one of the directories had the same time stamp each day. That prompted me to check the Task Scheduler. There I found 2 entries for Mozilla:
Firefox Developer ... At 10:26 PM on 8/1/2021 - After triggered, repeat every 07:00:00 indefinitely.
Firefox Developer ... At 7:26 AM every day

Now it made sense. 8/1/2021 is the day I started using the Web Developer. A little more research revealed that Firefox started using cert9.db/key4.db with V58. Because I am using V56, the temporary files did not get deleted.

J.A. Coutts