Synopsis

For several weeks, Firefox displayed an annoying "kind of" popup on startup (advertising seen.de, a domain I had visited when looking for a house for vacation). It wasn't a real window, resisted inspection, and disappeared after a couple of seconds. Today, I finally got around to debug it - I ran a simple

 find ./ -type f -print0 |xargs -0 grep -i seen

in my Mozilla profile directory to detect all files containing the string "seen", and lo and behold, one hit was in serviceworker.txt

Turns out serviceworker.txt apparently contains informations about all running service workers (surprise!). After some Ecosia'ing, I found a tip that recommended opening

about:debugging#workers

And indeed, there was a service worker registered to seen.de. I unregistered it - hopefully, this will get rid of the popups once and for all.