This means nothing - Rachel Brand is the Associate AG so if Rosenstein and Sessions are fired, she becomes Acting AG. If Sessions resigns, then the President can pick another Senate-confirmed official.
No, I think if Sessions is fired, Trump is stuck with three Obama appointees in a row, and the Senate Judiciary has already warned that their calendar is full for the rest of the year.
Unless Session resigns Trump is stuck with Rosenstein or a Obongo holdover as acting AG for who knows how long. So it looks like Trump wont even be able to get someone in there to get Mueller under control. Just great, what a mess. The recusal is the gift that keeps on giving.