It's a very useful tool, many thanks to the developers! However, it's unfortunately quite unreliable and not working most of the time. I haven't figured out yet what influences the malfunction or what brings it back to working again. Once this is fixed, 5++ stars!

Edit: I tried the add-on in Chrome and there it seems to be 100% reliable, so it seems like it has to do something with the firefox version.

Edit Edit: Thanks so much to the developers for fixing the reliability on Firefox, it works like magic now!