Very strange. How did you narrow down the problem to Firefox though?

Do you close Firefox or still leave it running before you shutdown your Macbook?

Thank you for your response. Answering both of your requests: - I narrowed it down by trying a lot of different options, but I don't remember now. I have been in this predicament for a year. After uninstalling Firefox, the mac shuts down as it should. I was keeping an eye on Firefox updates, and every two month or something I do the test again, download Firefox, use it and close the Mac. The first day works well, the second day again comes the problem so I uninstall. - I have tried both options: close Firefox before shoot down and keep it open for the Mac to close it at shoot down.

Note I use 4 pin tabs: gmail, calendar, google keep and toodledo and actually this is the primary reason I use Firefox, because of the logic behind pin tabs.

Note (Jan 4, 2021): I also remember how I did narrow down to Firefox, through one error report by Mac OS after starting back the macbook pro, it said something like "waiting on Firefox" at one point. Also: I made a mistake with the Macbook pro model on my original post, is mid 2012.