About two days after I installed this add-on, today suddenly my computer started freezing time and again, freezing so much I had to shut him down by unplugging the power cable!
I started to examine everything, hardware, software and it showed that Firefox was the culprit!
As nothing else was different or new or updated than the days before, I concluded, minerBlock must have done that freezing.
I deleted it and - oh wonder - Firefox works fine again.