If I'm understanding the two expansions correctly, then "The Dragon" doesn't add any new adventure cards, only dragon cards related to the scales. Furthermore it involves the return of the Dragons driven off by the crown's maker, while "The Firelands" involves the return of the Ifrit who helped forge the Crown. So thematically and mechanically, how well do these two expansions mesh?

I don't think they necessarily detract or enhance each other that much. From a flavor text point of view, the Ifrit would seem to come, somehow, from the same place as the Dragons. They work fine enough together, especially the way we play, but do not really support or distract from each other.

Mechanically, it's easy! Mix the Firelands Adventure Cards and Spells and Endings and Characters in with the rest, draw, and play!

Only restriction is (as with all Talisman games) is that if you play the standard dragon game (drawing scales every turn, using crown token, etc.) is that you can't use alternate endings.

Don't worry so much about the theme, lol.