Thank you very much Jason W.
I tested firefox behaviour again and do come to the conclusion, Firefox probably is communicating an 'autoupdate' which in reality didn't take place.
After updating the Firefox extension from a local dir - internect connection cut - Firefox, tries to communicate that 'autoupdate' too, even though it couldn't have taken place as connection was cut. See screenshot:
So I think it's a mere Firefox communication problem and not a problem you need to care for.
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By the way, I like your Shiretoko, but the name 'Firefox' is far better known, and creating a ready made VDI install for Windows users I think they will prefer something they know.