Since I upgraded from FF128-esr to FF140-esr I always get the "Firefox is being updated by another instance" This is really driving me crazy, as I checked / did / .... everything (and even more) I found on the internet related to that. Nothing helps ! Creating and using a new default profile didn't help Finally I ended up uninstalling FF completely (making sure no -zero- leftovers remained in ProgramFiles, AppData a.s.o.), and reinstalled FF140.7.0-esr freshly. After installation I started FF immediately with the '''created default profile''' (only existing one) --> SAME THING : "Firefox is being updated by another instance" And this is really a FF140 thing, when I go back to FF128, on the same computer, same profiles, ...., the problem is gone. I would like to know''' in detail what checks are really''' done by FF to decide to give that "Firefox is being updated by another instance"