If a firm enters or leaves, there is always a net benefit of zero.
For Firm A, its dominant strategy is to enter the market, because 1 is greater than -2.
For firm B, its dominant strategy is also to enter the market because -1 is greater than -3.
Firm B would prefer both firms to leave the market so it can get to zero.
But, in this model, it can’t do that because it know if A enters, it will have to enter or face the costs of -3.