Time and Seasons:
There is no day or night in Muspellheim that the average traveler can understand. The Sun and Moon are entirely occluded by smoke, but the light of the fires creates a constant orangey-red sky, rather. The inhabitants actually do have a way of telling time and season, but they don't explain that to outsiders. To the traveler, there may as well be no time or season. Muspellheim spins closest to our world, ironically, at the winter solstice.
To say that Muspellheim is the Burning Land is quite literal. A good percentage of it is molten lava rock, and much of the rest is constantly aflame. Attempting to fly over it from most of its borders is nearly impossible, due to the smoke and fumes and long distances where you don't dare land anywhere. The general opinion is that unless you are a fire-etin, Muspellheim is uninhabited and uninhabitable, and no one can journey there anyway.
There is, however, a small portion of the Fiery Realm - perhaps less than a twentieth - where human beings can walk around, with caution. It is the small part of Muspellheim that is coastline; there are beaches of black sand and lava rocks, and bubbling hot springs, and holes in the earth where small fires spring up. Here the fire-etins keep their homes - huts made of shiny black rocks piled and melted together - and they use their human forms here. They are perfectly capable of traveling anywhere else in their world in fiery form, and indeed fire-giants all seem to be more comfortable in fire-form than flesh-form.
Supported By RavenKaldera