To fire someone based on the spot of the seniority list is some form of discrimination and normally age discrimination is the most obvious one.
I think most airlines would like to fire the top end of the seniority list based on their salaries that are often about 4 to 5 times as high as a junior FO including pension payments. So in that case you have to fire 4 or 5 FOs in order to retain the same amount of cash compared to someone who will be retiring in a few months time.
I don't think it's gonne be a case of just following the 'last in first out' rule. It's gonne be more about salaries and the fleet you are currently flying. Looking at the early retirement of the 74's for example. Re-training onto a different aircraft type is expensive as well.
It's going to be messy for the upcoming few years and jobs will be lost, but again I don't think the seniority list will be the main thing for job losses.