nothing is changed for the context menu in this update, I checked again and everything is fine, please try to remove and install the extension again

EDIT# 1: please write version of Firefox, Extension and PotPlayer. Also try with portable Firefox (to check with a clean profile) https://portableapps.com/apps/internet/firefox_portable

EDIT# 2: it's a Firefox bug (latest versions), it's not about extension 1.9.20 update, you can install older versions of extensions and you will get the same problem