Does tiramisu firm up in the fridge? I am making my first tiramisu, using this recipe from Gourmet (with high ratings on Epicurious). My mascarpone mixture looks terribly soft to me. 

As per recipe, I foamed 4 yolks with 80 ml sherry (I had no Marsala) and 95 g sugar at 55°C. The yolks were from recently bought M-sized eggs, there were no L-size in the supermarket. The volume increased a lot, but the foam itself was runny. I mixed in the 450 g mascarpone (the egg mass was still warmish), and it seemed to dissolve (I hope I didn't melt it). The result was still foamy and runny. Then I folded in the 240 g whipped cream. The final consistency is similar to egg whites beaten to stiff peaks. It is a foam, and not runny, but very aerated, and I suspect that, if left on a heap (instead of a bowl), it will flow flat over time. It is very unlike the cream layer in tiramisu I've had before. 

Is this normal? Will the cream harden in the fridge? Will the ladyfingers soak up some moisture? (They are coffee-dipped in this recipe). Or should I put in some gelatine to make sure it will keep shape? Or some other thickener? I have xanthan, but don't know how to incorporate it, the mass won't survive a mixer on full speed, and it will clump if not perfectly dispersed.
Your recipe doesn't specify 55°C, and I'd be surprised if 5–8 minutes over barely simmering water only gets that hot. Indeed, checking for sources:


McGee, in On Food and Cooking, says:


  When the temperature reaches 120°F/50°C, high enough to unfold some of the yolk proteins, the mix thickens, traps air more efficiently, and begins to expand. As the proteins continue to unfold and then bond to each other, the foam rises into fluffy mounds. The key to maximally light zabaglione is to stop the heating just when the foam teeters on the cusp between liquid and solid. Further cooking will produce a stiffer, denser, eventually tough sponge as the proteins over-coagulate


so that leads to the conclusion that 55°C wasn't hot enough, as it hadn't yet reached "the cusp between liquid and solid".
Hans-Dieter Belitz, Werner Grosch, and Peter Schieberle say in Food Chemistry:


  Egg yolk can be whipped into stable foam only at higher temperatures (optimum 72°C), the volume increasing about sixfold in the process. Above the critical temperature, the volume falls and the proteins coagulate. The protein coagulation is prevented by reducing the pH value, e.g., by the addition of acetic acid. This effect is used in the production of highly stable sauces


so that'd imply you want 72°C, or maybe higher due to the acid (wine) present. As a side benefit, that'd also pasteurize the eggs.


In summary, I think you should have cooked it hotter.

I doubt it is going to firm much in the fridge, hopefully it is stable enough to not further liquify.