Longtime reader and lurker, just recently signed up because I feel the need to talk about the latest strip. ^_^
I totally feel for Fi (although, yes, as many people have said, her biphobia is one definitely unappealing facet of her character) at this point. Who here hasn't gotten angry and depressed over losing someone they loved, even when it's "just" a really, really horrible break-up? I think Fi is entitled to voice her pain and anger, but I'm also glad that Mary is doing her job by slapping some sense into Fi. (The fact that it was literal, and the depiction, makes it pretty funny, too!)
Fi might not have a responsibility as Lia's jilted lover to stop the Mara, but definitely as the Ruler of Nod, Fi has a responsibility to keep her subjects safe. (If she doesn't, would that make her "as bad as Sadako", at least in the whole "don't care about the people of Nod, gonna go angst now"?)
So they should, at least, go after the Mara for that reason. I hope that Mary's argument runs along those lines in the next page. ^_^