Well, both are easy enough objectives. The rub is finding a bag that does both at the price you desire.
For your wildland firefighting gig I should think most any bag rated to the mid-30s or higher would be fine. I mean, for the most part, if theres a forest fire going on youre not really apt to be in the snow with 30-degree temps, no? With that in mind, REIs Nooksack UL +35 ($159; rei.com) would be just about perfect. Its fairly light (two pounds, five ounces), compacts well, and is insulated with PrimaLoft, which performs very well in damp weather (in case of thunderstorms and the like). Its $159. And really, Marmots Pounder ($159; marmot.com) would do nicely as well even though its rated to 40 degrees. As its name implies, it weighs a mere pound. It also uses PrimaLoft insulation.
Making the leap from a hot summer in the West to winter in Alaska or the Tetons, working as a dog-sled guide, is a big one. Youre looking at temps of around 0 F and probably much lower. So your summer bag isnt really going to cut it. At a minimum, Id suggest something such as Mountain Hardwears Lamina -15 ($210; mountainhardwear.com), which uses a proprietary fill material to give you a fairly light (four pounds, 13 ounces) bag that can go below zero without sending your bank account into the same territory. But use it in the summer? No way. The North Faces Tundra -20 ($239; thenorthface.com) is another good cold-weather bag. It employs well-regarded Polarguard Delta and has a water-resistant shell.
I really think youre looking for two different bags here. You might shop around, try eBay or Craigslist, and see if you can find a deal somewhere.