In obsolete architecture terms the difference between firms and firms

is that firms is the principal rafters of a roof, especially a pair of rafters taken together while firms is the principal rafters of a roof, especially a pair of rafters taken together.

As nouns the difference between firms and firms

is that firms is plural of lang=en while firms is plural of lang=en.

As verbs the difference between firms and firms

is that firms is third-person singular of firm while firms is third-person singular of firm.