Earth and fire admire and exasperate each other: fire wants it now, earth wants it right. Used well, fire brings the push and earth makes it real — a formidable team. Otherwise you feel like brake and chaos to each other.
One holds steady (fixed), the other flows (mutable): the fixed one gives the mutable roots, and the mutable teaches the fixed that changing isn't losing. Complementary — as long as neither caricatures the other as 'stubborn' or 'flaky'.