^^^ Good advice here, although it's not an automatic that there's something wrong with the store you mention. Is it a local shop? Does it primarily specialize in ski gear during the winter? Do the folks there shave yet? Do they include a certified fitter? Do they start by measuring your foot in both dimensions and asking you questions about how and where you ski? Do they take out the liner and check the shell fit? Do they recommend a snug (< 1 1/2 fingers in the gap) shell? If yes to all, early symptoms of competence. Still would agree with a reality check with Bootfitters Forum.
FWIW, have tried both boots, own a pair of Dalbello Crosses. As stated, different widths and also very different lasts. Suggest a search here, lot on Dalbellos and one good thread on Firearrows. Dalbello is a cabrio (three piece) design, so will be optimized for softer snow, bumps, trees, variable snow, great heel hold down if you have narrow ankles, and unusually easy to get in and out of. Good lateral stiffness, flexier feeling forwards than a typical overlap design, so takes a little practice to pressure your tips if you ski like that. Maybe not the first call if you ski a lot of ice or hardpack. The Firearrow is a sort of half-cabrio, actually feels and flexes more like an overlap design, but is easy to get on and off like a three piece.
IMO if you like the cabrio flex pattern, and it fits your own foot, the Boss is a nice intermediate-advanced choice. But suggest you also try some classic overlaps. If the shop has the Firearrow, it probably has some other Nordica models. But a lot of other manufacturers make excellent overlaps. Does this shop carry Langes or Technicas or Salomons? If yes to more than one of these, then you have most of the last/foot shapes covered. Go try on some. Look for very snug, not cushy in store, particularly around heel and ankle. They will pack out, remember.