From time to time, Firefox might try to update itself and get itself stuck an a loop where every time you try and open it, it tries to upgrade itself, fails, closes and then you have to try and open it again and the same thing happens.
95% of the time, this problem is caused by Logitech webcam software.
I don’t know why, I don’t know how, but the problem has hit my system several times and every time after some googling, the suggestion to try and close my Logitech camera software comes up and as soon as I do, Firefox is able to upgrade itself and open fine.