A couple of minutes and a good coffee spent on an issue. The idb, a wrapper over IndexedDB was failing silently. An isolated demo was luckily showing something, it was A mutation operation was attempted on a database that did not allow mutations in the console.
What Google tells about this issue is that it's related to working in private mode. Which was definitely not my case. However, another very old discussion mentions a setting that turns off history. And yes, the setting is there, in Firefox. Go to Options, then Security and Privacy and then there's a switch that either turns on or turns off history tracking. For some reason it was turned off in my FF.
And yes, turning it on brings IndexedDB back to life.